PHOENIX, Arizona (Reuters) — Control of Congress was up for grabs early today after the United States midterm elections, with many of the most competitive races uncalled, leaving it unclear whether Republicans would crack Democrats’ tenuous hold on power.

In a critical win for President Joe Biden’s party, Democrat John Fetterman flipped a Republican-held US Senate seat in Pennsylvania, beating Republican celebrity doctor Mehmet Oz and bolstering his party’s chances of holding the chamber.

The mood at the White House improved as the night wore on, with once-nervous aides allowing smiles to creep on to their faces and saying early signs for Democrats were better than expected. On Twitter, Biden posted a photograph of himself happily congratulating some of the Democratic winners by phone.

In the House of Representatives, Republicans remained favoured to win a majority that would allow them to block Biden’s priorities while launching politically damaging investigations into his administration and family.

By early this morning, Republicans had flipped six Democratic House seats, Edison Research projected, one more than the minimum they need to take over the chamber.

Republicans will have the power to block aid to Ukraine if they win back control of Congress, but analysts say they are more likely to slow or pare back the flow of defence and economic assistance.

Stuart Cole, head macro economist at Equiti Capital, said partisan deadlock between a president and Congress and a Republican-controlled House would likely prevent any tax increases and limit government spending, which could mean interest rates will not have to rise as much to curb inflation.

House Republican Leader Kevin McCarthy had hoped to celebrate a resounding victory that would propel him into the top job of House Speaker.

Instead, he had to settle for a promise to his supporters: “When you wake up tomorrow, we will be in the majority and (Democratic Speaker) Nancy Pelosi will be in the minority.”

However, only 13 of the 53 most competitive races, based on a Reuters analysis of the leading non-partisan forecasters, had been decided, raising the prospect that the final outcome may not be known for some time.

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.

But Republican hopes for a “red wave” of victories faded as Democrats showed surprising resilience in several key races. Democrats were projected as the winners in 11 of the 13 close contests that had been decided.

“Definitely not a Republican wave, that’s for darn sure,” Republican US senator Lindsey Graham told NBC in an interview.

Pelosi said in a statement: “It is clear that House Democratic members and candidates are strongly outperforming expectations around the country.”

Republican former president Donald Trump, who took an active role in recruiting Republican candidates for Congress and is hinting at a third run for the presidency in 2024, had mixed results.

He notched a victory in Ohio, where author J.D. Vance won a Senate seat to keep it in Republican hands. But television host and heart surgeon Oz failed to win his Pennsylvania Senate race.

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

Meanwhile Florida governor Ron DeSantis, who could be a main Republican challenger to Trump in 2024, added to his growing national profile, defeating Democratic challenger Charlie Crist by nearly 20 percentage points, Edison projected.

Voter anger over the Supreme Court’s June decision to overturn the nationwide right to abortion helped Democrats to curb their losses.

The Senate was still a toss-up, with the pivotal battles in Arizona, Georgia and Nevada still in play.

The Georgia Senate race could end up in a December 6 run-off, possibly with Senate control at stake. Democrats control the 50-50 Senate with Vice-President Kamala Harris able to break any ties.

Thirty-five Senate seats, all 435 House seats and three dozen governors’ races were on the ballot.

More than 46 million Americans voted ahead of Election Day, either by mail or in person, according to data from the US Election Project, and state election officials caution that counting those ballots will take time.

High inflation and abortion rights were voters’ top concerns, exit polls showed. Crime, a major focus in Republican messaging in the campaign’s final weeks, also was a top issue.

Both parties notched victories in competitive districts.

Local officials reported isolated problems across the country, including a paper shortage in a Pennsylvania county. In Maricopa County, Arizona — a key battleground — a judge rejected a Republican request to extend voting hours after some tabulation machines malfunctioned.

The problems stoked evidence-free claims among Trump and his supporters that the failures were deliberate.

Scores of Republican candidates have echoed Trump’s false claims that his 2020 loss to Biden was owing to widespread fraud, raising fears among Democrats that they could interfere with the 2024 presidential race.

In Pennsylvania, Republican gubernatorial candidate Doug Mastriano, who sought to overturn the state’s election results after Trump lost, was defeated by Democrat Josh Shapiro. Democratic governors also fended off strong Republican challenges in Michigan and Wisconsin, two states likely to remain political battlegrounds in the 2024 presidential race.

Voters in California, Michigan and Vermont approved referendums enshrining abortion rights in their state constitutions. Deeply conservative Kentucky looked poised to reject a constitutional amendment that would have declared there was no right to abortion.

The primary issue weighing on Democrats was stubbornly high annual inflation, which at 8.2 per cent stands at the highest rate in 40 years.

A Reuters/Ipsos poll this week found just 39 per cent of Americans approved of the way Biden has done his job. Some Democratic candidates deliberately distanced themselves from the White House as Biden’s popularity languished.

Trump’s polling is similarly low, with just 41 per cent of respondents to a separate recent Reuters/Ipsos poll saying they viewed him favourably.