Republicans currently hold a 54-46 advantage over Democrats, meaning to gain control of the Senate, Democrats would need to pick up five seats if Donald Trump wins the White House or four if Hillary Clinton comes out on top.
Sen. Ron Johnson had been left for dead -- a first-term Republican facing a well-funded Democratic challenger in former Sen. Russ Feingold in a state Clinton is expected to carry.
But polls show Johnson has inched much closer to Feingold, and could pull off a major upset.
Johnson has shifted his campaign tactics with a mix of positive ads about himself and attacks against Feingold, and seems to have caught Democrats off guard. Buoyed by an influx of outside spending -- such as $2.2 million from Senate Majority Leader Mitch McConnell's super PAC -- and Trump outspending Clinton on TV in the state, Johnson has suddenly found himself down by just 1 point, according to a new Marquette University poll.
Still, both sides acknowledge that Feingold is the favorite heading into Election Day, especially given that no GOP candidate has won a Wisconsin Senate seat in a presidential year since 1980.
The race between incumbent Republican Sen. Kelly Ayotte and Democratic Gov. Maggie Hassan -- two popular women who each have been elected statewide more than once -- has swung back-and-forth for months.
Hassan worked hard to tie Ayotte to Trump and some of his most controversial comments, especially negative things he's said about women. Ayotte stumbled during an October debate by saying she considered Trump a role model for children. She took it back the next day, but that didn't stop Democratic groups from flooding the airwaves with her awkward response.
Hassan got help from Democratic heavy hitters when she sat on the stage with Hillary Clinton, Sen. Elizabeth Warren, D-Massachusetts, and Sen. Bernie Sanders, I-Vermont, who won the state's Democratic presidential primary.
But Republicans have dropped major dollars into TV, radio and digital advertising that argued Ayotte would be a curb on a Clinton White House. The late money fortified Ayotte's position, making the race a true toss-up.
The race has tilted in favor of Democratic challenger Katie McGinty in recent weeks as Clinton's poll numbers have held steadily above Trump's in this key battleground state. That has created a challenging playing field for GOP incumbent Sen. Pat Toomey to win re-election.
Toomey's campaign has been marked by his refusal to either endorse Trump or fully condemn the GOP presidential candidate for his controversial statements and positions. This middle position has made it difficult for Toomey to win the forceful backing of Trump supporters or the support of moderate voters.
McGinty, a former environmental aide for President Bill Clinton, has repeatedly blasted Toomey for what she describes as his waffling on Trump.
For his part, Toomey has worked the Philadelphia suburbs to appeal to moderate voters -- especially women -- to help power him to victory. But the national headwinds may end up being too tough for him to overcome.
Sen. Richard Burr's candidacy has been a constant cause of stress for GOP leaders. They don't think he has worked hard enough to define his Democratic opponent, Deborah Ross, or make the case that he should be re-elected.
GOP groups have poured money into an effort to define Ross as an out-of-touch liberal elite, picking apart her record working as a top attorney for the North Carolina chapter
of the liberal American Civil Liberties Union.
Scrutiny over Burr's private comments at a recent fundraiser
-- suggesting some gun owners may want to incite violence toward Clinton -- has put him on the defensive in the closing days of the campaign.
This race could tip with the presidential contest in North Carolina. In 2012, Mitt Romney narrowly defeated President Barack Obama -- but polls have consistently shown Clinton leading Trump in the state.
Sen. Marco Rubio's loss in the Republican presidential primary turned out to be the Florida GOP's gain, as Rubio -- who had planned to retire -- was coaxed by GOP leaders into running for a second term.
Rubio's presence turned his opponent, Rep. Patrick Murphy, from a top-tier recruit into a candidate Democrats considered leaving for dead before a super PAC tied to Harry Reid made a last-minute play in the state.
Murphy has been stung by reports that he inflated his business resume. Rubio, meanwhile, has struggled to distance himself from Trump, and has faced constant questions from Democrats over whether he'd serve a full six-year term rather than running for president again in 2020.
A Murphy win would be a signal of a Democratic wave, since Rubio is a favorite to win a second term.
Sen. Roy Blunt could be one of the few Republicans thanking Trump on Election Night if he holds onto his seat.
Democrat Jason Kander, currently Missouri's secretary of state, has emerged as perhaps his party's strongest candidate of the 2016 cycle. Under fire from the National Rifle Association over his support for gun control measures, Kander -- an Afghanistan veteran and former member of the Army National Guard -- cut one of the campaign's most memorable TV ads. Kander calmly puts together an AR-15 rifle in the spot -- while blindfolded.
However, Kander doesn't have the same advantage Democrats in battleground states do. Trump, polls show, is on course for a double-digit win in Missouri -- which means Blunt would have to lag badly behind his own party's nominee to lose his seat.
Republicans long looked certain to hold the seat vacated by retiring Sen. Dan Coats.
Then, this summer, the Democratic nominee -- former Rep. Baron Hill -- suddenly dropped out, and the party replaced him on the ballot with Evan Bayh. The popular former governor and senator was his party's best-known figure in Indiana, and was still sitting on a $10 million campaign war chest left over from his retirement in 2010.
Republican Rep. Todd Young, backed by the US Chamber of Commerce and super PACs tied to Mitch McConnell, has savaged Bayh for post-Senate lobbying work and raised questions about Bayh's residency. Bayh, stuck permanently on the defensive, has seen his numbers collapse in the polls, turning Indiana into a toss-up.
Democrat Catherine Cortez Masto, Nevada's former attorney general, is seeking to become the first Latina in the Senate. Republican Rep. Joe Heck, meanwhile, is attempting to pull off what could be Republicans' lone pickoff of a Senate seat in the 2016 election cycle.
Hovering over this race is retiring Senate Minority Leader Harry Reid, long the dominant Democrat in Nevada politics.
This race -- like Nevada's presidential contest -- could be decided by Latino turnout. Democrats hope Trump's controversial remarks about Mexicans and immigrants will sink him with the key constituency, and they've worked for months to link Heck to Trump.
Closely-watched but outcome anticipated:
First-term Sen. Mark Kirk long has been considered the most vulnerable Republican running for reelection this cycle and he appears poised to lose Tuesday to Democratic Rep. Tammy Duckworth, an Iraq war veteran.
While Kirk was an early critic of Trump's -- something that might have helped him in this blue state -- the former Naval intelligence officer couldn't distance himself from his own controversial comments during the campaign, such as last week, when he challenged Duckworth's statement that her family's military roots traced back to George Washington, since she also has descendants from Thailand. Kirk apologized, but his fate probably was sealed already.
The race featured two disabled candidates. Kirk suffered a struck stroke four years ago and regularly uses a cane or wheelchair. Duckworth lost both her legs in battle and uses a wheelchair.
Sen. John McCain appears set to win his sixth Senate term after fending off a respectable campaign from Democratic Rep. Anne Kirkpatrick in this red state that is beginning to tilt blue.
McCain, a former Republican presidential nominee who is the chairman of the Armed Services Committee, had a troubled relationship with Trump, who early in his campaign criticized the senator for having been a prisoner of war during Vietnam.
While McCain rose above those comments and for many months backed his party's nominee -- in part because McCain knew many of his base voters supported Trump -- he ultimately withdrew that backing after the "Access Hollywood" video surfaced in which Trump bragged about sexually assaulting women.
GOP Sen. Chuck Grassley, appears set to win re-election to a seventh term after overcoming a challenge from Democrat Patty Judge, a former lieutenant governor who was considered a serious contender when she first entered the race.
Grassley, the chairman of the Judiciary Committee, faced heavy criticism from Senate Democrats at the time for blocking President Barack Obama's Supreme Court nominee Merrick Garland. While the sharp criticism on the Senate floor may have stung, Republicans in Iowa backed Grassley's tough stance and his candidacy.
Iowans also are impressed by his reliability and durability, reflected in 7,653 consecutive votes he has cast in the Senate dating back to 1993.
Grassley was an early supporter of Trump's and unlike some Senate Republicans candidates who are now facing possible defeat, he never waffled on that.
Sen. Rob Portman was considered an endangered Republican when popular former Democratic Gov. Ted Strickland decided to run against him, although polls show Strickland's bid has fizzled.
Portman, a former U.S. Trade Representative in the George W. Bush administration, initially backed current Ohio Gov. John Kasich's campaign for the White House, but when Kasich withdrew, Portman quickly threw his support behind Trump. However, after the "Access Hollywood" video, Portman withdrew that support and said he would write-in Trump's running mate, Indiana Gov. Mike Pence, instead.
Before the controversial video surfaced, Portman, a soft-spoken veteran of Washington and proud policy wonk, said Trump's unorthodox candidacy helped his re-election race by energizing the GOP base.
Other notable races:
Twenty-four people, including former KKK leader David Duke, have jockeyed for the open Senate seat being vacated by GOP Sen. David Vitter, who ran for governor but lost. With such a crowded field, no one is expected to break the 50% threshold of the vote to avoid triggering a December 10 runoff between the top two finishers.
That means it's possible we won't know which party will control the Senate until then.
Polls show Republican state treasurer John Kennedy and Democrat Foster Campbell, a member of the Public Service Commission leading the race but others -- including two GOP congressman -- are in the hunt. While Louisiana is largely a red state, it also elects Democrats statewide, such as in the most recent governor's race. The political world will turn its gaze south if the Senate is at stake in a runoff.
Democratic State Attorney General Kamala Harris, who is expected to win Tuesday, would be only the second African-American women elected to the Senate in history -- the first being Carol Moseley Braun of Illinois, who served in the 1990s.
Harris would join two other black men in the Senate, Tim Scott, R-South Carolina, and Cory Booker, D-New Jersey, which will mark the first time ever three African-Americans have served in the Senate at the same time, according the Senate Historian's Office.
Sen. Chuck Schumer holds a commanding lead over his Republican opponent Wendy Long and is expected to cruise into his fourth term. Schumer, who has spent decades steadily climbing the ranks on Capitol Hill, is set to take over the helm of the Senate Democrats replacing Harry Reid, who is retiring. The only question is whether Democrats pick up enough seats to make him majority leader -- a very powerful post -- or minority leader if they fall short.
Rand Paul went to great lengths to preserve his ability to run to keep his Senate seat at the same time he was making an ultimately unsuccessful run for the White House.
Paul is expected to be re-elected to the chamber, where he will join former presidential rivals Ted Cruz, R-Texas, and, possibly, Marco Rubio, R-Florida, if he wins Tuesday. The three young conservatives spent that last two years circling each other like political warriors as they tried to outmaneuver one another and position themselves to win the GOP presidential primary.
They are sure to remain vocal critics of the status quo and ambitious -- each no doubt readying for future runs for the presidency.