The Senate consists of 100 lawmakers serving six-year terms; every election year about one-third of Senate seats are up for grabs. The Democrats currently hold a razor-thin majority in the upper chamber with 50 seats in their party's control. Remember that Democrats only need 50 seats to retain power, because Vice President Kamala Harris casts the tie-breaking vote.
In this year's midterm elections, Republicans are defending 21 seats, while Democrats are defending just 14 seats though three of the four closest Senate races this cycle have a Democrat incumbent.
The Senate remains a toss-up. Fox News' Power Rankings show 47 seats going to the Democrats and 49 to the Republicans, leaving four crucial toss-up races to decide control of the Senate: Arizona, Georgia, Nevada and Pennsylvania.
Arizona, Georgia and Nevada are all being defended by incumbent Democrats, while Pennsylvania is an open seat due to the retirement of longtime Republican Sen. Pat Toomey last year.
Tech venture capitalist and Republican nominee Blake Masters is challenging Democrat incumbent Sen. Mark Kelly, a retired astronaut, who has held the seat since defeating Martha McSally in a 2020 special election.
Kelly's election was the second time since 1988 that Arizona elected a Democrat to the U.S. Senate, and Kelly is fighting tooth and nail to hold onto his narrow lead in the polls putting him ahead by 3 points, according to a RealClearPolitics average. Though Kelly has not trailed Masters in any poll to date, his margin over the GOP nominee has slimmed in recent weeks.
Kelly has been able to overcome Biden's underwater approval ratings with his strong popularity among Arizona voters, 49% of Arizona voters hold a favorable opinion of the incumbent Senator according to a recent NY Times/Siena poll.
Masters, on the other hand, has struggled with a likability problem throughout the race with a majority of voters holding an unfavorable opinion of the GOP nominee.
Democratic incumbent and Baptist pastor Sen. Raphael Warnock, who was elected in a 2021 special election, faces off against scandal-ridden, Trump-endorsed Herschel Walker on November 8.
Walker, the GOP nominee, has faced allegations from two women who claim the former football player paid for their abortions, contradicting his pro-life stance on the campaign trail. Walker has denied both allegations.
Biden won Georgia by the smallest margin of any state in the 2020 presidential race, and has been noticeably absent from the Peach State throughout the midterm elections.
Georgia is unique from other battleground races in that a candidate must reach a simple majority — at least 50 % of the vote — to declare victory. As it stands right now, a Georgia runoff, tentatively set for December 6, is looking likely. Neither candidate up to this point has been able to hold a majority of votes in recent polls.
In Nevada, Democrat incumbent Sen. Catherine Cortez Masto is facing Republican challenger Adam Laxalt in a race that many political pundits are saying will determine the majority party in the Senate.
Cortez Masto was first elected in 2016 as the first Latina Senator and is considered one of the most vulnerable candidates up for re-election this cycle.
Laxalt, a former Nevada Attorney General, was endorsed by Trump and, if elected, would be the third generation in his family to serve in the Senate, following his grandfather and father.
Though the Silver State has been trending Democrat in recent presidential years, the GOP have benefited from a growing number of Hispanic voters identifying as Republican, as well as a large working class population, a voter bloc particularly afflicted by Covid-19 lockdowns as the economy is heavily dependent on tourism from Las Vegas and Reno.
Pennsylvania is another Senate race that has political analysts scratching their heads where a pair of atypical candidates has garnered nationwide attention.
Fetterman, the current lieutenant governor and former small-town mayor, is a hoodie-wearing progressive who beat out his more moderate primary challenger with charisma that has resonated with Pennsylvania's working class.
Best known for his celebrity appearances as Dr. OZ on the Oprah Winfrey Show, Oz has to sought characterize Fetterman as a freeloading career politician with a questionable record as former mayor of Braddock, Pennsylvania.
Fetterman's health has been in question since his stroke in May, though his disastrous debate performance in late October has failed to abate concerns that the Lieutenant Governor may not be fit to serve in the U.S. Senate.
Despite Fetterman's obvious health problems, Oz has trailed in polls by a narrowing margin as he has struggled with allegations of being a carpetbagger from New Jersey and an out-of-touch celebrity.
Fox News' Remy Numa contributed to this report.