Washington DC - With control of the US Senate hanging in the balance, both Republicans and Democrats fought hard to gain ground or maintain their seats.

Democratic President Joe Biden and Republican former president Donald Trump loomed over several of the Senate races.

Democrats currently control the Senate, one of two chambers in Congress, by a razor-thin margin. The net loss of a single seat in the 100-member Senate would flip control of the chamber to Republicans.

Republicans could block much of Biden's legislative agenda by taking control of either the Senate or the House of Representatives, where they are widely expected to make gains.

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Here is a rundown of who won the most competitive races, based on results tabulated by The Associated Press.

Democratic Sen. Michael Bennet prevailed over moderate Republican and construction company owner Joe O’Dea, winning his third full term representing Colorado.

Bennet is known as a policy-focused senator willing to negotiate bipartisan deals. He sought his party’s presidential nomination in 2020 but failed to break through in a crowded field of Democratic candidates with bigger national profiles.

O’Dea’s victory in the Republican primary was thought to have tightened the race, after Democratic groups spent money to boost his more conservative opponent.

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O’Dea took traditionally conservative fiscal policy stances but opposed Trump and backed rights to abortion and same-sex marriage.

Republican Sen. Marco Rubio defeated a challenge by Democratic Rep. Val B. Demings to win a third term. Demings, the former police chief for the city of Orlando, highlighted her law enforcement background while challenging Rubio in the Republican-leaning state.

Republican Sen. Charles E. Grassley, who was first elected in 1980, the same year Ronald Reagan first won the presidency, was re-elected to an eighth term. Grassley (89) beat a challenge from Democrat Mike Franken, a retired US Navy admiral.

Trump had gone to Iowa to campaign for Grassley in the Republican-leaning state.

Democratic Sen. Maggie Hassan defeated Republican Don Bolduc to win a second six-year term. Polls had showed a tightening race in the final weeks of the campaign.

Republican Rep. Ted Budd defeated former North Carolina state Supreme Court Chief Justice Cheri Beasley to win a US Senate seat in the Republican-leaning state. Budd will replace retiring Republican Sen. Richard M. Burr.

Budd had previously served six years in the House of Representatives. Republicans considered holding the seat necessary to win control of the Senate.

Republican J.D. Vance, a venture capitalist and author of the bestselling memoir Hillbilly Elegy about his hardscrabble upbringing and his mother’s struggle with addiction, beat Democratic Rep. Tim Ryan in the Republican-leaning state.

Vance will replace retiring fellow Republican Sen. Rob Portman.

Former President Trump endorsed Vance and held a rally for Vance the day before Election Day.

Democratic Lt. Gov. John Fetterman defeated television doctor Mehmet Oz in one of the closest-watched Senate races this year.

Fetterman claimed a US Senate seat left open by retiring Republican Sen. Patrick J. Toomey. The victory is a significant boost to Democratic chances of holding the Senate.

Fetterman overcame the challenges of a stroke in May that impaired his hearing and speech to edge out the Trump-endorsed Oz, whom Democrats derided as a millionaire carpetbagger from the neighboring state of New Jersey trying to buy the open Senate seat.

More than $250 million was spent on the race, according to a WESA radio station report.

Republican Sen. Mike Lee won re-election in Utah, defeating independent former presidential hopeful Evan McMullin, who said he would not align himself with either party if elected to the Senate.

McMullin focused his campaign on threats to democracy, trying to appeal both to Democrats and to Republicans critical of Trump. But Lee, who had the backing of Trump and the Senate Republican establishment, was able to win in the traditionally red state.

Democratic Sen. Patty Murray beat back a challenge from Republican Tiffany Smiley to win a sixth term representing the generally liberal state in the north-west United States.

Biden won Washington state by 19 points in 2020, indicating that the Republican challenger had an uphill battle to oust Murray even with voters concerned about inflation and souring on Biden.

While many results have already been announced, control of the Senate is not yet clear, with several key races still up in the air. The contest between Georgia's Democratic incumbent Sen. Raphael Warnock and Republican challenger Herschel Walker is expected to go to a runoff.