The Democratic party’s hopes of reclaiming control of the US Senate faded after two Republican senators, Lindsey Graham of South Carolina and Joni Enrst of Iowa, won their re-election battles.
A Republican candidate also won back a Senate seat in Alabama but Democrats ousted an incumbent in Colorado as the parties waited for the results of other races that will be crucial to the balance of power in the upper chamber of Congress.
Mr Graham, a one-time Trump critic who became a staunch ally of the US president, defeated Democratic challenger Jaime Harrison in the most expensive Senate race in US history.
Mr Harrison, a 44-year-old former state party chairman, raised a record $109m in his bid to oust Mr Graham, 65, who called Donald Trump a “race-baiting, xenophobic, religious bigot” in the run-up to the 2016 election before later changing his tune.
A third of the Senate is up for grabs this year with Republicans currently controlling 53 of the upper chamber’s 100 seats versus the Democrats’ 47.
Democrats need a net gain of three seats to take the Senate if Joe Biden is elected president and a gain of four Donald Trump is re-elected because the US vice-president can cast a tiebreaking vote.
Democrats are expected to retain their control of the 435-seat House of Representatives.
In Colorado, Democrat John Hickenlooper, the state’s former governor, defeated Republican incumbent Cory Gardner, who had been seen as one of the GOP’s rising stars.
Initial returns in Arizona suggested Democrat Mark Kelly, a former astronaut and husband of Gabby Giffords, the congresswoman who stepped down after an assassination attempt in 2011, was on course to oust incumbent Republican Martha McSally.
But in Alabama, incumbent Democrat Doug Jones lost his race against Republican challenger Tommy Tuberville, a former college football coach.
Republican Senate majority leader Mitch McConnell handily won re-election in Kentucky, defeating his Democratic challenger Amy McGrath, a former Marine fighter pilot. The McGrath campaign raised a sizeable $88m, according to the Center for Responsive Politics, compared to the $55m brought in by Mr McConnell’s campaign.
Republicans also breathed a sigh of relief in Iowa, where Ms Ernst edged out a challenge from Democrat Theresa Greenfield.
Many states may take days or even weeks to finalise their vote counts from Tuesday and at least one Senate race will not be decided until January.
In Georgia, a special election to determine whether former Wall Street executive Kelly Loeffler, a Republican, will keep the seat she was appointed to fill in December last year after Johnny Isakson stepped down due to health concerns, will advance to a run-off, to be held on January 5.
Ms Loeffler, a former Intercontinental Exchange executive, will face off against Democrat Raphael Warnock, pastor of Atlanta’s Ebenezer Baptist Church, in the run-off.
Mr Warnock and Ms Loeffler finished first and second, respectively, in a “jungle primary” on Tuesday that had 20 candidates on the ballot.
Georgia is the only state in the US where both Senate seats are up for grabs this cycle. Results are still being counted in the other race, where Jon Ossoff, a 33-year-old Democrat, is challenging 70-year-old Republican incumbent David Perdue. If neither man garners more than half of the votes — a Libertarian party candidate is also in the running — that contest will also head to a run-off on January 5.