The Republican Party will win a majority in the US House of Representatives, according to projections by American media outlets.
The Associated Press and NBC News called California's 27th congressional district for the Republicans, giving them 218 seats in the House.
But the party will have a slim majority, with Joe Biden's Democrats holding at least 210 seats.
The US president congratulated his opposition, saying he would work with them to deliver for Americans.
"The American people want us to get things done for them," Mr Biden said in a statement issued by the White House.
"I will work with anyone — Republican or Democrat — willing to work with me to deliver results for them."
The Associated Press said seven seats remained in doubt.
Senator Mitch McConnell, who held off a challenge to his position as Republican leader in the Senate, said the Republicans were "full of energy and ideas".
"We're going to fight Democrats' recklessness and promote our commonsense conservative vision to improve families' lives and strengthen America," he wrote on Twitter.
But the Republicans' expected slim majority fell short of the "red wave" the party had hoped for going into last week's midterm elections.
Democrats held the Senate and managed to stem their losses in the House, in an unusually strong midterm performance from the party that holds the presidency.
But the result means the Democrats' legislative agenda will be frustrated, with the GOP now able to block bills.
Republicans will also control the agendas of powerful committees, and it is likely they will curtail or shut down the January 6th committee investigating the attack on the Capitol by supporters of former president Donald Trump in January 2021.
Mr Trump launched his own 2024 presidential campaign on Tuesday US time, telling supporters inside a Mar-a-Lago ballroom he would "save the American dream".
But he faces opposition from other Republican heavyweights, including Florida Governor Ron DeSantis, and former vice-president Mike Pence.
Mr Trump's daughter Ivanka Trump, who served as one of his senior advisors when he was president, confirmed she would not be returning to politics shortly after her father's announcement.
ABC/wires
