United States Senate control hangs in the balance while Republicans move closer to securing a majority in the House of Representatives, a day after Democrats outperformed expectations and avoided a Republican "red wave" in midterm elections.
The Senate contests in Nevada and Arizona, where Democratic incumbents were seeking to hold off Republican challengers, were as yet undecided late on Wednesday, with thousands of uncounted ballots that could take days to tally.
If the parties split those races, the Senate's fate would come down to a Georgia runoff election for the second time in two years, after Edison Research projected neither Democratic incumbent Raphael Warnock nor Republican Herschel Walker would reach the 50 per cent necessary to avoid a December 6 one-on-one rematch.
Republicans were closing in on the 218 seats needed to wrest control of the House from Democrats, with 208 now in their column, Edison Research projected.
But 23 of the 53 most competitive races, based on a Reuters analysis of the leading nonpartisan forecasters, were still pending as of Wednesday afternoon.
Even a slim House majority would let Republicans hem in Democratic President Joe Biden during his next two years in office, blocking legislation and launching potentially politically damaging investigations.
Speaking at a White House news conference, Biden vowed to work with Republicans and said he understood voters are frustrated despite Democrats' surprisingly competitive campaign.
"The American people have made clear, I think, that they expect Republicans to be prepared to work with me as well," Biden said.
He also reiterated his intention to run for re-election in 2024 and said he would make a final decision early next year.
The election fell far short of the sweeping victory Republicans sought, with Democrats avoiding the kind of heavy midterm defeat that often plagues sitting presidents.
The results suggested voters were punishing Biden for presiding over an economy hit by steep inflation, while also lashing out against Republican efforts to ban abortion and cast doubt on the nation's vote-counting process.
Poor performances by some candidates backed by Donald Trump - including Walker - signalled exhaustion with the kind of chaos fomented by the Republican former president, raising questions about the viability of his possible 2024 White House run.
Biden had framed Tuesday's election as a test of US democracy at a time when hundreds of Republican candidates embraced Trump's false claims that the 2020 presidential election was stolen.
A number of election deniers who backed Trump's claims were elected to office on Tuesday, but many of those who sought positions to oversee elections at the state level were defeated.
"It was a good day, I think, for democracy," Biden said.
Fears of violence or disruption by far-right poll watchers at voting stations did not materialise.
Jen Easterly, head of the US Cybersecurity and Infrastructure Security Agency, said she saw no evidence any voting system was compromised.
Control of the Senate would give Republicans the power to block Biden's nominees for judicial and administrative posts.
But in a critical win for Democrats, John Fetterman flipped a Republican-held US Senate seat in Pennsylvania, beating Trump-backed retired celebrity surgeon Mehmet Oz and bolstering his party's chances of holding the chamber.
Democrats also had their share of embarrassments, as New York Representative Sean Patrick Maloney, the chairman of the committee charged with re-electing House Democrats, conceded he had lost his own race.
If the Republicans do take control of either chamber, they plan to seek cost savings in the social security and Medicare safety-net programs and make permanent tax cuts enacted in 2017 that are due to expire.
Republicans also could engineer a showdown over the debt ceiling to extract major spending cuts and could pare back aid to Ukraine.
The party that occupies the White House almost always loses seats in elections midway through a president's first four-year term, and Biden has struggled with low public approval.
"In this climate we should have done better," said Rob Jesmer, a former head of the Republicans' Senate campaign arm.