These are the Senate seats at play in the 2018 midterms. Republicans control the red states; Democrats hold the blue ones.

Story highlights

Republican Sen. Dean Heller's stances on health care have hurt him

Democratic Sen. Heidi Heitkamp has benefited from the President's praise

Washington CNN  — 

President Donald Trump is an influential figure all over the 2018 Senate map. The problem for Republicans: He doesn’t seem interested in using his power for the party’s good.

From labeling a top Democratic target a “good woman” to tacitly approving of his former White House chief strategist Steve Bannon’s aggressive courtship of Republicans to run in the primary against sitting senators, the Trump factor is proving problematic as midterm election season heats up.

Here’s a look, more than a year out, at the 10 Senate seats most likely to switch parties in the November 2018 midterms:

1. Nevada

Incumbent: Republican Sen. Dean Heller

Primary date: June 12

In a health care battle that hurt Republicans politically, Heller’s reversals and contortions were uniquely damaging.

His maneuvers – he was against one GOP health care bill, then for another one – looked like a result of the pinch Heller is feeling from both sides. On the right, Danny Tarkanian – 0-for-5 in the general election in his runs for office but 4-for-5 in Republican primaries – says Heller hasn’t sufficiently supported Trump. On the left, Democratic Rep. Jacky Rosen says he’s been there when Trump and congressional Republicans needed him.

Nevada was a rare swing state to go Hillary Clinton’s direction in 2016. Heller only won by a nose in 2012 – and the health care debacle makes this state the most likely to flip next year.

2. Missouri

Incumbent: Democratic Sen. Claire McCaskill

Primary date: August 7

Running for re-election as a Democrat in a state Trump won by 19 points is tough enough. Now, McCaskill faces the challenger national Republicans had hoped to recruit into the race: state attorney general Josh Hawley, who officially launched his campaign last week.

Worse news for McCaskill: At the outset, Hawley seems to have the backing of both the McConnell and Bannon crowds – making a competitive primary that would siphon away resources less likely.

McCaskill starts from a major financial advantage. She raised nearly $3 million in 2017’s third quarter and has $7 million in the bank, to Hawley’s $820,000 third-quarter haul. Still, at this stage in the 2018 cycle, she looks like the Democratic incumbent in the most jeopardy.

So why isn’t she first on the list? Because of this: You can’t yet name a single tough vote Trump and congressional Republicans have forced her to take. It’s a reality that helps every Democratic incumbent on this list.

3. Arizona

Incumbent: Republican Sen. Jeff Flake

Primary date: August 28

If the question was which senator is most likely to lose, Flake would be first – by a lot. His support from Republicans has so thoroughly collapsed that the question that looms largest is simply whether he’ll even appear on the ballot.

Democrats have the candidate they hoped for in Rep. Kyrsten Sinema. But Arizona still favors Republicans, and whoever emerges from the GOP’s primary will stand a chance at keeping the seat – even if it’s Kelli Ward, whose history of controversial statements would be fodder for endless attack ads.

For now, Ward is the only Republican running against Flake. And several groups’ internal polls there show Ward crushing him. But the possibility of other candidates entering before the late-August 2018 primary, and the bigger question of Sen. John McCain’s health, add heavy doses of uncertainty to this race.

4. Indiana

Incumbent: Democratic Sen. Joe Donnelly

Primary date: May 8

Donnelly is a ripe target: He gambled entering the 2012 race against Sen. Richard Lugar, got lucky when Lugar lost in a primary to state treasurer Richard Mourdock, and then hit the jackpot when Mourdock flubbed a question about pregnancies resulting from rape in a debate just before the election.

None of the leading Republican candidates this time around – Reps. Todd Rokita and Luke Messer and state Rep. Mike Braun – are likely to commit such a gaffe.

This is another race that features two major themes: (1) It’s happening in a state where Trump cruised in 2016, and (2) Republicans are heading into what could be an expensive, brutal primary.

The first factor lands Indiana high on the list. The second one keeps it from climbing higher – for now.

5. West Virginia

Incumbent: Democratic Sen. Joe Manchin

Primary date: May 8

It’s getting lonely for Manchin in West Virginia.

Trump won the state by 42 points. Then the governor who’d been elected as a Democrat stood with Trump at a rally and declared that he was becoming a Republican.

Manchin, a former governor himself, knows his state. He’s played the politics of the Trump era well, playing footsie with the administration over a Cabinet post and sounding open to compromise on issues like taxes. Like every other Democrat on this list, he’s helped by Republicans failing to make him pay any price at all for voting against their bills. But serious GOP challengers are lining up to take him on, and the D beside his name might be too much for Manchin to overcome in such a conservative state.

6. North Dakota

Incumbent: Democratic Sen. Heidi Heitkamp

Primary date: June 12

Want a 10-second explanation of why Trump drives Republicans whose job is to expand the party’s Senate majority crazy?

Cue up the tape of Trump, who had flown Heitkamp to his event in North Dakota in early September on Air Force One, pointing her out and saying: “Everyone’s saying: What’s she doing up here? But I’ll tell you what: Good woman.”

What better gift could Trump give a Democrat running for re-election in a state he won by 36 points?

Republican Rep. Kevin Cramer, who is thinking about challenging Heitkamp, was on stage for that moment, and got no special mention. State Sen. Tom Campbell is in the race. But Heitkamp raised more than $1 million in the third quarter, has almost $4 million on hand and the President seeming to like her is going to matter.

7. Ohio

Incumbent: Democratic Sen. Sherrod Brown

Primary date: May 8

The 2016 election ended Ohio’s status as the marquee swing state – with Trump’s eight-point margin of victory there making places like North Carolina, Arizona and Georgia look like potentially better targets for national Democrats.

When it comes to connecting with voters’ sense of economic anxiety, the populist, anti-trade Brown has a lot in common with Trump. He also heads into a likely matchup against state treasurer Josh Mandel, who Brown already beat head-to-head in 2012.

This race should be a good test of whether Trump’s popularity transfers to other Republicans – or whether it was really his economic message that won the day in Ohio.

8. Montana

Incumbent: Democratic Sen. Jon Tester

Primary date: June 5

Tester is raising lots of money – $1.2 million in the third quarter, with more than $5 million on hand – and railing against “Washington,” not Trump.

Yes, Montana was a state Trump won by 20 points. But Gov. Steve Bullock also proved last year that a Democrat can win statewide here. And Republicans haven’t yet been able to cast a man with a flat-top who still farms on the weekends as an out-of-touch liberal elitist.

The GOP primary bears watching – in part because it’s shaping up as a proxy war between interior secretary Ryan Zinke, whose wife is involved with Troy Downing’s campaign, and Sen. Steve Daines, who is aligned with state auditor Matt Rosendale.

9. Wisconsin

Incumbent: Democratic Sen. Tammy Baldwin

Primary date: August 14

Baldwin is all that’s standing in the way of Republicans’ total takeover of Wisconsin’s statewide seats – and she’ll be on the ballot at the same time the engine that drove Gov. Scott Walker’s victories in two gubernatorial races and a recall election is trying to win him a third term.

But the Republican primary field is unsettled. State Sen. Leah Vukmir and businessman Kevin Nicholson could pose serious challenges.

One big question: Was Trump’s win in 2016 a surprise that’ll quickly be reversed, or the tipping point in what had been Wisconsin’s slow rightward drift?

10. Florida

Incumbent: Democratic Sen. Bill Nelson

Primary date: August 28

This is all about Gov. Rick Scott, a Republican who can throw tons of his own money into a Senate race that Trump and other party leaders desperately want to see him enter. If he does, this becomes a high-profile race in a marquee swing state, and has all the ingredients of a close, expensive and brutal contest. Scott’s handling of Hurricane Irma – and particularly a nursing home where 12 patients died – would come under serious scrutiny.

For now, this race barely makes the cut. But if Scott gets in, expect it to jump much higher.

Honorable mention

Two more Democrats, Pennsylvania Sen. Bob Casey and Virginia Sen. Tim Kaine, are among the safest swing-state incumbents. And Michigan Sen. Debbie Stabenow appears to be in good position – though rap-rock musician Kid Rock (ballot name: Robert Ritchie) wants people to think he might run against her.

In New Jersey, Sen. Robert Menendez is on a different kind of hot seat – in the middle of a corruption trial.

If a true anti-Trump wave breaks against the Republicans, Texas Sen. Ted Cruz could find himself in a competitive race against Rep. Beto O’Rourke. And whichever Republican is nominated to replace Sen. Bob Corker in Tennessee – right now, Rep. Marsha Blackburn looks like the front-runner – could face a tough test.

One that should be on your radar: Mississippi. Republican Sen. Roger Wicker is likely to face a Steve Bannon-backed primary challenger. State Sen. Chris McDaniel, who nearly knocked off Sen. Thad Cochran in a primary in 2014, is seriously considering the race. Democrats wonder if the GOP primary might turn this state into competitive ground just like Alabama, where controversial former judge Roy Moore faces a surprisingly serious competitor in Democrat Doug Jones in a December special election.