As Republicans won a majority in the US Senate, control of the US House of Representatives remained unclear early Wednesday morning , with many of the most competitive races still uncalled.

With Republicans controlling the Senate, and Trump ahead in the electoral vote count, the stakes for control of the House have grown, since it could serve as either the sole Democratic check on a Trump presidency, or as the final Republican win that delivers a united pro-Trump government.

Republicans had made progress as the first results in the battle for the House were announced. By 2am ET, Republicans were ahead 189 to 166, according to the Associated Press. North Carolina Republicans had won three seats previously held by Democrats, while Democrats flipped seats in Alabama and New York.

The US may have to wait days to learn who won the House, given that it took more than a week to make that call in 2022.

All 435 members of the House face re-election, with Republicans looking to expand their narrow majority after two chaotic years in power.

Republican House speaker Mike Johnson was re-elected to a fifth term in the House, and is seen as likely to continue his leadership role if the Republicans are able to keep control of the chamber, while Elise Stefanik, another loyal Trump ally and the highest-ranking woman in House Republican ranks, won a sixth term in New York.

But control of the lower chamber appears to be a true toss-up, as Democrats only need to win five more seats than they did in 2022 to regain their majority.

In an election marked by Republicans’ intense anti-trans rhetoric, Delaware voters elected the first openly trans member of congress, Democrat Sarah McBride, 34.

Democrats picked up their first additional seat in upstate New York, where Democrat John Mannion defeated Republican congressman Brandon Williams, according to the Associated Press.

So far, both Democrats and Republicans have picked up seats due to redistricting, the process of adjusting district lines to keep up with population changes, with Alabama Democrat Shomari Figures winning a district that had been changed to ensure fair representation for Black voters, while Republicans in North Carolina flipped three districts that had been reshaped by the Republican-controlled state legislature.

Democrats also flipped two seats in New York state, with Democrat Josh Riley, an attorney, beating incumbent Republican Marc Molinaro, and John Mannion, a Democratic state senator, defeating incumbent Republican Brandon Williams.

Ten of the most hotly contested house races are in California, where Democrats need to flip at least one Republican seat in order to secure a majority, according to the Associated Press.

Without control of the House, the winner of the presidential race – whether it be Kamala Harris or Donald Trump – will face significant hurdles in implementing a legislative agenda. Election forecasts suggest either party could end up with a majority of just a few seats, which could recreate some of the problems of the 118th Congress.

In 2022, the “red wave” that Republicans had promised failed to materialize, leaving the party with a House majority of just four seats at the start of 2023. That tight margin allowed a small group of hard-right Republicans to wreak havoc on the speakership race, forcing Kevin McCarthy to endure 15 rounds of voting before capturing the gavel.

But just nine months later, McCarthy was ousted from the speaker’s chair following a revolt staged by eight members of his own conference. McCarthy’s removal kicked off weeks of chaos, with the House at a complete standstill until Johnson, then a relatively unknown lawmaker, was elected to lead the chamber.

In recent months, Republicans have had to pitch themselves to voters for another two years in power after overseeing the most unproductive Congress in decades. Democrats have attempted to capitalize on Republicans’ legislative record as they look to rebuild a majority in the House, warning voters about the dangers of continuing the “dysfunction” in Congress.

“You’ve all seen it from day one, with 15 rounds of speaker elections [and] threats of shutdowns,” Congresswoman Suzan DelBene, chair of House Democrats’ campaign arm, told Axios in August. “Those are the things that drive people at home crazy.”

If Democrats can flip just a handful of Republican-held seats, Hakeem Jeffries, the current House minority leader, is expected to take over as speaker. But despite Republicans’ checkered legislative record, Johnson has voiced confidence that his party will maintain control of the House.

“I believe we’re going to keep and grow the House majority, win the Senate and win the White House as well, and we’re going to put Donald Trump back into office,” Johnson said in September.

Regardless of who wins a House majority, the new Congress will immediately face a highly consequential task when members are seated in January: certifying the results of the presidential election. In 2020, Trump supporters infamously attacked the Capitol in an unsuccessful effort to disrupt the congressional certification of Joe Biden’s victory, and lawmakers have sparked concern about the possibility of similar political violence after election day.

Read more of the Guardian’s 2024 US election coverage