The race to decide who has control of the US Senate could take weeks as election officials warn of long voting counts in Arizona and Nevada.
Both states are crucial to deciding whether Republicans or Democrats hold the balance of power - but President Joe Biden was upbeat at the outcome so far.
"It was a good day, I think for democracy, and it was a good day for America," he told reporters.
The result has so far been less of a drubbing than many Democrats had been fearing, despite losing control of the House.
Republican house minority leader Kevin McCarthy made promises his party was willing to work with Democrats - so long as it suited them.
"Republicans will work with anyone who's willing to join us to deliver this new direction that Americans have demanded," he said.
The balance of power could remain undecided until the run-off in Georgia next month - a result of neither candidate there reaching the 50% threshold.
The state is a toss-up between Democratic candidate Raphael Warnock, and former footballer and Trump-endorsed candidate Herschel Walker - who has faced accusations of paying for abortions while claiming to be pro-life.
"If they win, I should get all the credit," Trump said ahead of the elections.
"And if they lose, I should not be blamed at all."
His predictions of a sweep of far-right candidates did not eventuate, and he's now facing a potential challenger to the Republican presidential nomination in 2024.
"We will never, ever surrender to the woke mob. Florida is where woke goes to die," Ron DeSantis claimed, who's just been returned to a second term as Florida governor.
He smiled as the crowd chanted "two more years" - a reference to his potential running for president before his term as governor expires.
Trump is said to have been livid at the failure of his selected candidates to do better and at the news that he may face competition for the Republican presidential nomination.
"President Trump had said he'd be making an announcement on November 15th, next Tuesday," his advisor Jason Miller said.
"I'm advising the president to hold off until the Georgia race," he said.
But not going ahead with the "major announcement" could be embarrassing for the former president.
Whichever way Republicans fall, it's likely to cause a split.
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