Even a slim majority in the House of Representatives would let Republicans hem in Democratic President Joe Biden during his next two years in office, blocking legislation and launching potentially politically damaging investigations.

But they appeared to be far short of the sweeping “red wave” victory they had sought, as Democrats were avoiding the kind of heavy midterms defeat that often plagues sitting presidents of either party.

Control of the Senate remained unclear, as Democrats picked up a seat in Pennsylvania but awaited results from Nevada, Georgia and Arizona that would define the outcome.

Another competitive race broke in Republicans’ favour, as Edison Research projected that incumbent Senator Ron Johnson would win re-election in Wisconsin.

The results appeared to show voters punishing Biden for presiding over an economy hit by steep inflation, while also lashing out against Republican efforts to ban abortion.

Poor performances by some candidates backed by Donald Trump indicated exhaustion with the kind of chaos fomented by the former Republican president, raising questions about the viability of his possible run for the White House in 2024.

“I think his ego is just too big to handle,” said Yvonne Langdon, 75, as she cast her ballot for Republican candidates in Michigan on Tuesday.

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 from him.

A number of election deniers who backed Trump’s claims were elected to office on Tuesday, but fears of violence or other major disruptions by far-right poll watchers at voting stations did not materialise.

By early Wednesday, Republicans had flipped a net six Democratic House seats, Edison Research projected, one more than the minimum they need to take over the chamber. That number could go up or down as more final results roll in.

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 retired celebrity surgeon Mehmet Oz and bolstering his party’s chances of holding the chamber.

The Georgia race appeared to be headed for a runoff vote on December 6 because both the Democratic and Republican candidates were falling short of the 50 per cent needed for victory.

The White House seized on the lighter-than-expected losses. “Never underestimate how much Team Biden is underestimated,” White House Chief of Staff Ron Klain said on Twitter.

But 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 Congress, 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.

Trump, who took an active role in recruiting Republican candidates for Congress, had mixed results.

He notched a victory in Ohio, where “Hillbilly Elegy” author JD Vance won a Senate seat to keep it in Republican hands.

But Oz lost his Pennsylvania Senate race and Doug Mastriano, another Trump ally, was defeated in the state’s governor’s race.

Trump allies also were struggling in Arizona, Georgia and Nevada Senate races, where ballots were still being counted.