While the race for the White House has dominated national headlines, control of the U.S. Senate also is at stake on Tuesday.
There are 54 Republicans and 44 Democrats now in the Senate, with two independents, including former presidential candidate Bernie Sanders of Vermont, who caucus with Democrats.
On Election Day, 24 seats held by Republicans, and 10 held by Democrats, are on the ballot.
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[NATL] Highlights From the 2016 Campaign Trail
To take control of the Senate, Democrats need to gain four seats if Hillary Clinton wins the presidency (and VP Tim Kaine could cast a tie-breaking vote), or five if she loses.
Here are key Senate races to keep track of Tuesday night:
Illinois: Polls show Republican incumbent Mark Kirk trailing Democratic Rep. Tammy Duckworth. Kirk has held the seat since 2010. He's become the most vulnerable incumbent in the race, dealing with the effects of a 2012 stroke, and facing a strong opponent in Duckworth.
Duckworth, a veteran who lost both legs in Iraq, raised more money than Kirk.
Kirk has made waves more than once during this year's election.
In June, he renounced his support for Republican presidential candidate Donald Trump.
Last month, he sparked controversy when questioning the Duckworth family's military service during a debate after she said her family has served going back to the American Revolution.
"I had forgotten that your parents came all the way from Thailand to serve George Washington," Kirk said.
Duckworth's mother is a Thai immigrant. Her father served in World War II and has lineage traced back to a family member who fought in the American Revolution.
Kirk later apologized.
Wisconsin: Russ Feingold, a former Democratic senator who lost his seat in 2010, has a small lead in the polls against Republican incumbent Ron Johnson.
If Feingold beats Johnson, it will be the first time since 1934 that a senator who was voted out came back six year later to defeat a former rival, NBC News reported.
Pennsylvania: A close race is unfolding as Republican incumbent Pat Toomey is up against Democrat Katie McGinty, a former chief of staff to Gov. Tom Wolf. While Democrats portray Toomey as being too conservative, McGinty hasn't always connected well with voters.
Toomey hasn't endorsed or disavowed Trump. When pressed on the issue during a debate, Toomey said, "I don't think my constituents care that much how one person is going to vote."
Indiana: The race is shaping up to be tight as the state's Republican incumbent, Dan Coats, is retiring. Running for his seat are former Democratic senator Evan Bayh and Rep. Todd Young, a Republican.
Bayh, the son of longtime senator Birch Bayh, energized Democrats when he announced a run to reclaim the seat he gave up in 2010. However, his opponent and Young's allies have painted Bayh as an opportunist who cashed in on his Senate experience by working at a Washington, D.C. lobbying firm. A story from early October detailed the former senator's efforts to find a private sector job during his final year in the Senate.
Missouri: Republican incumbent Roy Blunt is facing Missouri Secretary of State Jason Kander, a Democrat. According to Real Clear Politics' polling average, Blunt, who has held the seat since 2010, holds a narrow lead over Kander, a former military intelligence officer.
Nevada: With Senate Minority Leader Harry Reid retiring, polls show a close race shaping up between Rep. Joe Heck (R) and Catherine Cortez Masto (D), the state's former attorney general.
While the race is a chance for Republicans to pick up a seat held by Democrats, Trump's controversial comments combined with the state's high Hispanic population have given Democrats hope.
New Hampshire: The Senate race between Sen. Kelly Ayotte (R) and Gov. Maggie Hassan (D) has been getting national attention as both presidential candidates and their surrogates have campaigned in New Hampshire. Ayotte found herself in a difficult position in early October, after saying that she would call Trump a role model for children. While she later said she "misspoke," Hassan's campaign seized the comment as an opportunity, calling it "a revealing moment that she [Ayotte] cannot take back." Polls show Ayotte with a slim lead against Hassan.
North Carolina: Democrats are hopeful that former state representative and American Civil Liberties Union lawyer Deborah Ross will defeat Republican senator Richard Burr. Republicans have been using Ross' record as a lawyer at the ACLU in an attempt to discredit her. Polls predict a close race.
Florida: Senator and former presidential candidate Marco Rubio (R) is fighting for his seat against Rep. Patrick Murphy (D). Rubio first said he would not seek re-election, but reversed that decision after dropping out of the presidential race, much to Republicans' delight. According to polls, Rubio has the edge.
While Rubio is known for his rocky relationship with Trump and frustration with the Senate, his opponent has had missteps. Murphy's been accused of touting pieces of his resume too much, and relying on his wealthy family for donations, NBC News reported.
Other Races to Watch and the Comey Effect:
Two other races that are important to watch, but are likely to remain in Republican hands, are in Arizona and Ohio.
Sen. John McCain (R) is working to hold off a challenge from Rep. Ann Kirkpatrick (D) in Arizona. McCain has held the seat since 1986 and is doing well, according to polls. Kirkpatrick has come down on McCain for his support of Trump. Though he originally endorsed Trump, McCain later renounced him.
In Ohio, polls show Sen. Rob Portman (R) leading his opponent, Ted Strickland, a former Democratic governor. Strickland has made some gaffes during the election, including saying that Justice Antonin Scalia’s death "happened at a good time."
If Democrats win the Senate and, in a longshot, the House, it will be the first time since 1952 that both chambers of Congress flipped during a presidential election year.
Another interesting possibility in this election: The number of women in the Senate will likely increase from 20, with females on the ballot in several states.
And while races are tight in multiple states, some Democrats are concerned about fallout from FBI Director James Comey's letter to the Senate Judiciary Committee announcing the FBI was reviewing a new set of emails in the probe into Hillary Clinton's private email server. The emails were found in the federal investigation into former New York congressman Anthony Weiner's sexting scandal.
The FBI review ended nine days later with Comey telling lawmakers the review was over and had not changed his original determination. But Democrats are concerned about the damage to down-ballot candidates, from an emboldened GOP and possible dampened enthusiasm among Democrats.