After Supreme Court Justice Ruth Bader Ginsburg’s death in September, Collins was the first Republican senator to break from party ranks and oppose moving forward with a replacement before the election. Collins ultimately voted against the confirmation of Amy Coney Barrett to the Supreme Court.
Prior to that, Collins had criticised Trump on occasion, but was unwilling to break from the party and vote against Trump in the Senate impeachment trial, and was a key enabler of the 2018 Senate confirmation of Trump’s conservative Supreme Court nominee, Justice Brett Kavanaugh.
Collins’s endorsement of Kavanaugh made her a target for Democrats, pro-choice advocates and women’s groups. Planned Parenthood, a reproductive health organisation that had previously supported Collins, endorsed her challenger, Democrat Sara Gideon, 48, who has gotten a big boost in funding from outside groups.
Gideon is well-known in Maine as speaker of the state House of Representatives and leads Collins in opinion polls.
Steve Daines, Montana
Steve Daines, 58, who previously served in the US House of Representatives, is running for a second term in the Senate. He faces Montana’s Democratic governor Steve Bullock, who has garnered high approval ratings in the state.
Bullock, 54, joined the Senate race after abandoning a long-shot presidential bid, and was recruited by both former President Barack Obama and Senate Minority Leader Chuck Schumer.
While Trump handily won Montana in 2016, polls show Daines with only a slight edge over Bullock and independent analysts rate the race a toss-up.
Both candidates have shattered fundraising records for the state, with Daines bringing in $13m and Bullock $11m as of the end of June.
Thom Tillis, North Carolina
Thom Tillis, 60, a former speaker of the North Carolina House of Representatives, is running for his second term against Democratic challenger Cal Cunningham, a former state senator and Iraq war veteran.
Early on, Tillis had questioned some of Trump’s decisions, including his emergency declaration to build a wall along the southern border with Mexico. But he became one of the president’s strongest defenders during the Senate impeachment trial.
The race blew up into one of the most dramatic in the country when Cunningham’s candidacy was tripped up by a sex scandal after he holding a significant polling lead for months. His double-digit lead dwindled throughout October.
Trump has strong support among rural voters in North Carolina, as he does throughout the US South, and Tillis’s campaign is betting Trump will help win over swing voters on the economy and immigration.
Lindsey Graham, South Carolina
Senator Lindsey Graham, one of Trump’s closest allies in Congress, was last re-elected to the Senate in 2014 with more than 55 percent of the vote. But this year, he is favoured to win by only a small margin against Democrat Jaime Harrison, who is running a powerful campaign backed by staggering fundraising numbers.
Recent polls show Graham with a slight lead over Harrison, based partly on his role in overseeing Barrett’s confirmation. Once a Trump critic, Graham faces scepticism among conservatives over his recent conversion to Trump ally, while the loss of his former maverick persona has disappointed moderates, analysts say.
Some polling evidence suggests Graham’s stewardship, as chairman of the Senate Judiciary Committee, of Amy Coney Barrett’s Supreme Court confirmation has helped him late in the campaign.
Doug Jones, Alabama
Doug Jones is the most vulnerable Democrat up for re-election in the Senate. He won office in 2018 in Republican Alabama in a special election contest against former Alabama Supreme Court Justice Roy Moore, whose candidacy was crippled by sexual misconduct allegations.
Jones now faces a more formidable foe in Tommy Tuberville, 66, a former big-time US football coach who led Auburn University to legendary victories over in-state rival University of Alabama six times between 1999 and 2008.
While Tuberville leads in polls, Jones has out-fundraised him by a wide margin. Independent analysts see the race leaning Republican.
Gary Peters, Michigan
Gary Peters, 61, is running for a second term against Republican challenger John James, 39, a Detroit businessman and a former helicopter pilot in Iraq.
James, an African American who has spoken about his interactions with police, has shown himself to be a formidable fundraiser running on a platform of unity and bipartisanship.
Peters faced criticism in the battleground state for his vote to convict Trump on articles of impeachment.
Peters leads slightly in polling and spending by outside groups. Analysts give him an edge to win in November.