CNN  — 

The month of May brings the first Senate primaries of the year. Ohio voted first, followed by a near weekly stream of contests that will shape the midterm battle for control of the chamber. By the end of the month, the general election matchups in almost half of this year’s most competitive races will be set.

Republicans only need a net gain of one seat this fall to win the Senate, and so far, much of the intrigue about the May primaries has been on the GOP side. Former President Donald Trump has endorsed in all four Senate contests on the ballot in May that are expected to be competitive in the fall – Ohio, where his chosen candidate won on May 3, North Carolina (May 17), Pennsylvania (May 17) and Georgia (May 24). Those primaries have cemented his hold on the party because of the way so many GOP candidates, even those without his endorsement, have tried to appeal to him – in some cases, twisting themselves into almost unrecognizable versions of their former selves.

Trump’s support has played differently in each state. In Ohio, it helped “Hillbilly Elegy” author J.D. Vance win the primary, likely blunting the biggest attack attack on him – the candidate’s past criticism of Trump.

The former President’s backing of Mehmet Oz in Pennsylvania hasn’t had as clear of an immediate impact, especially considering voters may have already associated the two celebrity Republicans together. And the millions in negative advertising spent attacking Oz or former hedge fund executive David McCormick seems to have opened a lane for conservative commentator Kathy Barnette. One Trump adviser said the former President is feeling “nervous about Oz’s prospects,” CNN’s Gabby Orr reported, especially after his endorsed candidate didn’t win Nebraska’s gubernatorial primary on May 10.

Trump weighed in on the North Carolina primary nearly a year ago, but his backing of US Rep. Ted Budd didn’t clear the GOP field. In Georgia, however, his commitment to Herschel Walker likely kept other top-name Republicans out of the race, leaving the former NFL star with what’s expected to be a relatively easy path to the Republican nomination at the end of the month.

Democrats argue that this month’s messy and expensive GOP primaries will leave Republican nominees bruised heading into the general election. (Democrats also have a crowded primary in Pennsylvania, but it hasn’t attracted anywhere near the level of spending or vitriol as the GOP contest.)

After watching Republican candidates run to the right to try to win their party nods, Democrats are hoping to make general election matchups a contrast in candidates. But that gets harder to do if the national environment continues to work against the party. President Joe Biden’s approval rating was at 41%, with 59% disapproving, according to CNN’s latest poll, conducted by SSRS from April 28-May 1. Democrats know Republicans will tie them to him, which is one reason why a number of them have broken with the White House on lifting Title 42, a Trump-era public health authority that allows border authorities to turn migrants back to Mexico or their home countries.

Senate contests are increasingly becoming nationalized affairs, so while candidate quality does matter (see No. 10 on this list), Democrats face a tough challenge in holding their razor-thin Senate majority, regardless of which Republicans emerge from these May primaries.

CNN’s ranking of the Senate seats most likely to flip sees a few changes this month, all in Republicans’ favor. But the states holding May primaries remain in the same positions this month. The ranking is based on CNN’s reporting and fundraising and advertising data, as well as historical data about how states and candidates have performed.

1. Pennsylvania

Incumbent: Republican Pat Toomey (retiring)

Pennsylvania – a state Biden won in 2020 – remains the seat most likely to flip, but this month’s primaries will tell us a lot more about how competitive the contest to succeed retiring GOP Sen. Pat Toomey could be in November. Trump weighed in on the GOP field in early April, endorsing celebrity heart surgeon Mehmet Oz over former hedge fund executive David McCormick. While Oz is trying to capitalize on the former President’s support, McCormick and his allies are arguing that McCormick is the real conservative. “I like Trump … but not his Senate pick,” voters say in one recent McCormick ad.

Meanwhile, political commentator Kathy Barnette has emerged as a late electoral threat to both men. The three candidates were statistically tied in a May 3-7 Fox poll released last week, and the political arm of the Club for Growth announced a nearly $2 million investment to boost her candidacy. The anti-abortion Susan B. Anthony List and CatholicVote are also backing her.

The big question for the May 17 primary is whether Trump’s endorsement will be enough for Oz, who had been trailing in some polls earlier this spring. Although 22% of GOP primary voters expressed a preference for Oz in the Fox poll, compared with 20% for McCormick and 19% for Barnette – all within the margin of error – Oz’s negatives were high compared with his opponents. Forty-six percent had an unfavorable view of Oz, while 27% and 12% had unfavorable views of McCormick and Barnette, respectively.

When it comes to Trump’s support for Oz, 37% of Republican primary voters said it made no difference to them. In a Monmouth University survey released in late April, after the Trump endorsement, 69% of GOP voters said the endorsement did not change how they felt about Oz.

The Democratic primary was rocked Sunday, two days before the primary, with the news that the front-runner, Lt. Gov. John Fetterman, had suffered a stroke. “I had a stroke that was caused by a clot from my heart being in an A-fib rhythm for too long,” he said in a statement, adding that he was “feeling much better” and that his campaign “isn’t slowing down one bit.”

In the Monmouth poll, 44% of Democratic primary voters said they were very likely to back Fetterman – nearly double the percentage who said the same about US Rep. Conor Lamb, who’s also from western Pennsylvania. A recent ad from Fetterman, in which he vows to “take on Washington,” comes across as a general election message from a progressive trying to run as a populist outsider. But his primary opponents have trained their attacks on him in recent debates, especially over a 2013 incident in which he brought along his shotgun to confront someone who later turned out to be an unarmed Black jogger.

Lamb has made the argument for his electability. Advertising from his allies, including one spot that opens with footage of the January 6, 2021, insurrection at the US Capitol, highlights his success defeating “Trump Republicans” in his Pittsburgh-area House seat – a clear play for the suburban voters so important to the Democratic coalition.

While Fetterman has been seen as the favorite for the nomination, especially given his statewide name recognition and hefty fundraising, the news of his health scare adds an element of uncertainty to Tuesday’s primary.

2. Georgia

Incumbent: Democrat Raphael Warnock

The Peach State, which remains at No. 2, also has its primaries in May, but there’s less intraparty excitement here. Despite enduring attacks from GOP opponents, former NFL player Herschel Walker, who has the backing of both Trump and Senate Republican leader Mitch McConnell, shouldn’t have a problem clinching the nomination to challenge Democratic Sen. Raphael Warnock.

Having won a special election runoff in January 2021, Warnock is now seeking a full six-year term. He raised an astounding $13.6 million in the first three months of the year, which follows impressive earlier quarterly hauls, and is running direct-to-camera ads on issues with bipartisan appeal such as jobs, infrastructure and health care. Meanwhile, Democrats hope that Warnock and gubernatorial candidate Stacey Abrams will generate the base turnout they desperately need in a midterm year.

But Warnock is in serious jeopardy, even against a wildcard candidate like Walker, who faces a myriad of questions about his business background and allegations of threats he made against women. Much of the GOP establishment viewed Walker as a liability at the beginning of the cycle but have, publicly at least, come around, while his team has worked to keep him relatively on script. Democrats’ road to Senate control ran through Georgia last cycle, but it’s still challenging political terrain for them, especially in a midterm year with an unpopular Democratic President.

3. Nevada

Incumbent: Democrat Catherine Cortez Masto

The Silver State moves up two slots on this month’s ranking. Democratic Sen. Catherine Cortez Masto faces a tough race in a transient state that Biden won by 2 points in 2020. The President’s approval rating has sunk since then, and Nevada has some of the highest gas prices in the nation, which could play into the GOP messaging that’s trying to pin steep daily living costs on the party in power. Even if Cortez Masto manages to differentiate herself from generic Democrats, that effort could be overtaken by a bad national environment, especially if Republicans are able to make more inroads with Latino voters. It doesn’t help that the state Democratic Party is fractured.

Cortez Masto, the former chair of Senate Democrats’ campaign arm, outraised likely GOP nominee Adam Laxalt in the first quarter of the year – $4.4 million to $1.6 million. But money isn’t expected to be an issue for Laxalt, a former state attorney general – he succeeded Cortez Masto in that role – and grandson of a former senator and governor, who will benefit from GOP outside spending. Democrats argue that Laxalt, who co-chaired Trump’s 2020 campaign in the state, has gone too far down the election conspiracy road and that his anti-abortion positions will alienate voters. But that may not matter as much if voters decide it’s time for a change in Washington.

4. Arizona

Incumbent: Democrat Mark Kelly

Holding steady in fourth position is Arizona, where Democratic Sen. Mark Kelly is running for a full six-year term after winning a special election in 2020. He’s one of the best-positioned Democratic incumbents when it comes to his personal story and campaign coffers. But the former astronaut has a voting record now and is running in a purple border state that only narrowly backed Biden. Kelly is trying to stake out differences from the White House – such as his opposition to lifting Title 42. But regardless of any real separation between him and Biden, messaging those nuances is an arguably tougher job than it is for Republicans to blanketly tie him to an unpopular President.

Luckily for Kelly, the messy Republican primary field won’t be sorted until August. So while he’s facing attacks from the National Republican Senatorial Committee, he doesn’t yet have a well-defined opponent. Trump has teased that an endorsement is coming. It’s unlikely to be for state Attorney General Mark Brnovich, who’s been a target of the former President’s wrath for not doing more to investigate the 2020 election. Venture capitalist Blake Masters is backed by Peter Thiel, who’s also behind Trump’s chosen candidate in Ohio. But Masters also has the endorsement of the Club for Growth, which may not sit well with Trump given his differences with the group over the Ohio Senate primary.

5. Wisconsin

Incumbent: Republican Ron Johnson

GOP Sen. Ron Johnson finally stepped up his fundraising – he brought in about $6 million in the first three months of the year after announcing in January that he was breaking his pledge to only serve two terms. He’s been running positive spots, featuring families who say their terminally ill loved ones benefited from Johnson’s support for so-called Right to Try legislation.

That’s not why Wisconsin slides down two spots on this list. The senator remains unpopular – 36% of registered voters viewed Johnson favorably while 46% viewed him unfavorably, according to a Marquette University Law School poll released Wednesday. (In February, the same poll found him underwater by a slightly worse 12 points.) Half of voters in the April Marquette survey said the phrase “cares about people like me” did not describe Johnson. That explains why Democrats continue to believe this seat is a better pickup opportunity with Johnson running than had it been an open seat.

But Biden is also unpopular in Wisconsin. After winning the state with about 49% in 2020, his approval rating has hovered at about 43%, with some 53% disapproving, since last fall. The winner of the Democratic primary in August will likely be tied to Biden, especially with inflation emerging as a top concern, according to the Marquette poll, and worries over the coronavirus (about which Johnson has said plenty of controversial things) reaching a new low.

The Democratic field is still uncertain. Support for Lt. Gov. Mandela Barnes, who has been seen as the front-runner, declined slightly among likely primary voters in the Marquette poll, while support for Milwaukee Bucks basketball team executive Alex Lasry ticked up. State Treasurer Sarah Godlewski and Outagamie County Executive Tom Nelson remained in the single digits. Nearly half of those likely primary voters, however, said they hadn’t picked a candidate. Lasry and Godlewski are pouring their own resources into the race to go up on TV. Barnes, who raised $1.7 million in the first quarter of the year, has not yet gone on TV. And none of the candidates has emerged as a fundraising powerhouse the way other Democrats challenging unpopular Republicans (think Sens. Ted Cruz or Lindsey Graham) have in cycles past.

6. New Hampshire

Incumbent: Democrat Maggie Hassan

New Hampshire remains at No. 6 as the Republican field – upended late last year by Gov. Chris Sununu’s decision not to seek the nomination – develops. Republicans looking to challenge Democratic Sen. Maggie Hassan include state Senate President Chuck Morse, former Londonderry town manager Kevin Smith and retired Army Brig. Gen. Don Bolduc, who unsuccessfully sought the nomination for the state’s other Senate seat in 2020. And other Republicans have been throwing their hats in the ring ahead of New Hampshire’s late primary in September.

Hassan, a former two-term governor, is seeking a second Senate term in a state that, on paper, should be more favorable to Democrats than any other state on this list. Biden carried New Hampshire by 7 points in 2020. An unfavorable national environment, however, could hurt her reelection chances, even without an opponent of Sununu’s caliber. Hassan has come out against the Biden administration’s plan to lift Title 42, making her own trip to the border last month that angered several Latino leaders back home.

7. North Carolina

Incumbent: Republican Richard Burr (retiring)

We’ll soon find out if one of Trump’s earliest Senate primary endorsements – for US Rep. Ted Budd last June – results in victory. Budd must exceed 30% of the vote in the May 17 primary (which was delayed from March because of redistricting uncertainty) to avoid a runoff for the seat GOP Sen. Richard Burr is vacating. Budd’s strongest opponent is former Gov. Pat McCrory, but former US Rep. Mark Walker has the potential to siphon off support from the congressman. The conservative Club for Growth Action has been boosting Trump’s pick. One recent spot features Republican Lt. Gov. Mark Robinson praising Budd and knocking McCrory. “Pat’s a nice guy, but he’s no conservative,” Robinson says, admitting that he’d voted for McCrory in the past “but not this time.” McCrory and his allies are touting his record on immigration and arguing that Budd is “weak” on a host of issues.

On the Democratic side, former state Supreme Court Chief Justice Cheri Beasley has essentially cleared the primary field and has already pivoted to the general election. “Looking at Washington, I think both parties are doing the job wrong,” she says in a recent ad. Beasley outraised her potential GOP opponents in the first quarter of this year, but money won’t be enough in a state Trump won twice, especially in a midterm year, when Democratic turnout has often waned. Senate Leadership Fund, the GOP super PAC aligned with McConnell, has made ad reservations in the state to help the eventual nominee. The Democratic equivalent, Senate Majority PAC, left North Carolina off its list of initial reservations.

8. Florida

Incumbent: Republican Marco Rubio

Democratic Rep. Val Demings, a former Orlando police chief with a compelling story, continues to outraise Republican incumbent Marco Rubio, bringing in more than $10 million during the first three months of the year to Rubio’s $5.8 million. Her campaign recently announced a Hispanic outreach effort and a $3 million investment in a state bilingual coordinated campaign.

But while money is important in this expensive state, it won’t be enough to flip the seat. And none of the major outside groups have made ad reservations here, signaling that they view the race as less competitive. Demings already started at a disadvantage in a state where Trump expanded on his victory margin in 2020 from four years earlier, but her job would be even harder if the national mood continues to sour on Democrats this fall. She and Rubio have traded barbs on gas prices and immigration. Rubio’s campaign has tried to tie Demings to Biden’s decision to rescind Title 42. Demings’ team recently said she does not support lifting the measure “until there is a plan to put more boots on the ground and support our law enforcement officers at the border.”

9. Ohio

Incumbent: Republican Rob Portman (retiring)

The first major test of Trump’s endorsement in a Senate primary this year was a success for the former President. “Hillbilly Elegy” author J.D. Vance, who received Trump’s backing in mid-April, won the nomination on May 3 with about 32% of the vote, finishing ahead of former state treasurer Josh Mandel (24%) and state Sen. Matt Dolan (23%).

Vance’s public criticism of Trump from 2016 had been a treasure trove for his primary opponents and their ad-makers. The former President, though, was willing to look past that. And it paid off for Vance, who saw his support double in a Fox poll ahead of the primary. It was a topsy-turvy race, but it’s likely that Vance landed on top at just the right moment for it to matter.

Still, missing out on Trump’s endorsement didn’t stop the jockeying among some other candidates (and their allies) to tie themselves most closely to him. Club for Growth Action, for example, which had backed Mandel and is in an open spat with Trump, ran more clips of Vance’s past criticism of Trump. Another Club ad featured people, some of whom said “they love Trump,” questioning the former President’s judgment on endorsements past and present.

Only Dolan ran truly outside the Trump election conspiracy lane in the GOP primary. Interestingly, his stock had risen in polling right before the election, presaging his third-place finish ahead of candidates who had tried to tie themselves more closely to Trump, including former state party chair Jane Timken, who had the backing of retiring Sen. Rob Portman, and businessman Mike Gibbons.

Democratic Rep. Tim Ryan easily won his party’s nomination but starts at a disadvantage in this reddening state.

10. Missouri

Incumbent: Republican Roy Blunt (retiring)

It’s still all about Eric Greitens in Missouri – does the former governor, who resigned in disgrace in 2018, stay in the Republican race? Does he score Trump’s endorsement? And does he win the GOP nod in August? Those are the series of questions that will determine whether Missouri – a red state that shouldn’t otherwise be competitive – remains on this list.

Concerns about Greitens deepened in March after court documents revealed that his former wife had alleged he was physically abusive toward her and his children, which he denies. Greitens is no stranger to controversy, having resigned as governor amid allegations of sexual misconduct. Many of his primary opponents have called on him to drop out. Anxious Republicans would like to see their primary field thin, fearing that such a crowded pool of candidates could end up splitting the anti-Greitens vote and handing him a victory.

Some Democrats smell opportunity. Trudy Busch Valentine, a nurse and Anheuser-Busch beer heiress, jumped into the race this spring. She hasn’t said how much she’ll invest in her campaign, but should this race become competitive, Democrats may benefit from having a candidate with her own money to spend. But Valentine first has a primary to win – while one Democrat dropped out of the race and endorsed her, the top fundraiser, Marine veteran Lucas Kunce, doesn’t look like he’s going anywhere.

This story has been updated with additional details in Ohio’s and Pennsylvania’s Senate races.