Republicans have edged closer to a House majority while control of the Senate hinges on a few tight races after Democrats staved off an anticipated "red wave" of Republican gains in midterm elections.
Republicans have captured at least 210 House seats, Edison Research projected, eight short of the 218 needed to wrest the House away from Democrats and effectively halt President Joe Biden's legislative agenda.
While Republicans remain favoured, there were 33 House contests yet to be decided - including 21 of the 53 most competitive races, based on a Reuters analysis of the leading nonpartisan forecasters - likely ensuring the final outcome will not be determined for some time.
The fate of the Senate was far less certain. Either party could seize control by winning too-close-to-call races in Nevada and Arizona, where officials are tallying thousands of uncounted ballots.
The party in power historically suffers heavy casualties in a president's first midterm election and Tuesday's results suggested voters were punishing Biden for the steepest inflation in 40 years.
But Democrats were able to avoid the major defeat that Republicans had anticipated and were holding on in the close Senate battles in Nevada and Arizona.
Tuesday's results also suggested voters were lashing out against Republican efforts to ban abortion and to cast doubt on the nation's vote-counting process.
Biden had framed the election as a test of US democracy at a time when hundreds of Republican candidates embraced former President Donald Trump's false claims that the 2020 presidential election was stolen.
A split in the Senate vote would mean the majority would come down to a runoff election in Georgia for the second time in two years.
Democratic incumbent Raphael Warnock and Republican challenger Herschel Walker both failed to reach 50 per cent on Tuesday, forcing them into a one-on-one battle on December 6.
Even a slim House majority would allow Republicans to shape the rest of Biden's term, blocking priorities such as abortion rights and launching investigations into his administration and family.
Bide acknowledged that reality on Wednesday, saying he was prepared to work with Republicans.
A White House official said Biden spoke by phone with Republican House leader Kevin McCarthy, who announced earlier in the day his intention to run for speaker of the House if Republicans control the chamber.
"The American people have made clear, I think, that they expect Republicans to be prepared to work with me as well," Biden told a news conference.
If McCarthy is the next House speaker, he may find it challenging to hold together his fractious caucus, with a hard-right wing that has little interest in compromise.
Republicans are expected to demand spending cuts in exchange for raising the nation's borrowing limit next year, a showdown that could spook financial markets.
Control of the Senate, meanwhile, would give Republicans the power to block Biden's nominees for judicial and administrative posts.
A number of "election deniers" - those who support Trump's false claim that the 2020 presidential election was stolen from him - won on Tuesday but many who sought positions to oversee elections at the state level were defeated.
"It was a good day, I think, for democracy," Biden said.
Trump, who took an active role in recruiting Republican candidates, had mixed results.
He notched a victory in Ohio, where "Hillbilly Elegy" author J.D. Vance won a Senate seat to keep it in Republican hands. But several other Trump-backed candidates suffered defeats, such as retired celebrity surgeon Mehmet Oz, who lost a crucial Senate race in Pennsylvania to Democrat John Fetterman.
Meanwhile, Republican Florida Governor Ron DeSantis, who could challenge Trump in 2024, won re-election by nearly 20 percentage points, adding to his growing national profile.