Republicans have struck an early victory in the race to control Congress, with a formerly Democratic-aligned seat in West Virginia won by the state's current governor, Jim Justice.
In addition to the presidency, all 435 seats in the House of Representatives and 34 seats in the Senate are up for grabs on election day.
Republicans currently hold a majority in the House. The projected win by Justice takes the Senate to a 50-50 split. Results in other seats are pending, however.
The party that wins a majority on either side of Capitol Hill will have greater leverage to enact its own agenda, regardless of who is in the White House.
One party in control of the House, Senate and the White House would have broad power to pass laws and enact the president's partisan agenda.
The Senate seat in West Virginia flipped Republican, after the retirement of the former Democrat Joe Manchin, who frequently clashed with members of his own party before turning independent.
The winner, Justice, had himself been a Democrat before switching parties to Republican at a Trump rally in 2017.
Republicans were also projected to retain control of a Senate seat in Florida.
Incumbent Rick Scott, also a former governor, was first elected six years ago. He was running against former Miami-area Congresswoman Debbie Mucarsel-Powell, a Democrat who migrated from Ecuador as a child.
Speaking at a victory rally, Scott predicted that Republicans would seize the majority in the Senate, and that he would be selected as Senate Majority Leader.
"Florida is the centre of the Republican party of this country," he told the crowd. "Washington can learn a hell of a lot from what we've done right here in this great state."
Democrats held a seat in Delaware, a state that President Joe Biden represented as a senator for 36 years, with a victory for fourth-term Congresswoman Lisa Blunt Rochester.
In Maryland, the Democrats retained a seat vacated by Ben Cardin, with Angela Alsobrooks beating the popular Republican Governor Larry Hogan.
They will become the first black female senators to represent their respective states.
Other contests that would swing control of the upper chamber of Congress are taking place in Arizona, Pennsylvania, Montana, Michigan, Nevada, Nebraska, Texas and Wisconsin.
Both parties are also vying for control of the House, although the balance of power there will not be known for several days.
The vast majority of House elections are happening in "safe districts" - regions where one party is nearly certain to win. But a handful of other races in swing districts could determine who controls Washington DC.
House races that could swing the balance are taking place a wide variety of states, with closely watched elections happening in California, New York, Washington, Maine and Alaska.
