Non-partisan election forecasters predicted that Republicans are likely to pick up roughly 25 seats in the 435-seat House of Representatives, more than enough to win a majority. Analysts said Republicans also could pick up the one seat they need to win control of the Senate.

Republicans have blamed Biden's administration for rising prices and crime, two top voter concerns. But dozens of candidates also have echoed former President Donald Trump's baseless claims of fraud in his 2020 election defeat. Some of them could end up as governors or election administrators in battleground states and play a central role in the 2024 presidential race.

Early voters

More than 42 million Americans have cast early ballots ahead of Tuesday's midterm elections, surpassing the numbers from 2018, the US Elections Project had said on Monday.

All 435 seats in the House of Representatives are on the ballot, as well as 35 Senate seats, just over a third of the upper chamber. Votes will also be cast for governors in dozens of states, as well as thousands of other county and state positions.

Most of the 50 US states allow voters to cast ballots early, either in-person or by mail, a practice which became widespread during the 2020 presidential election, which was held at the height of the Covid-19 pandemic.

According to the US Elections Project, as of Monday there have been more than 19.3 early votes cast in-person and 22.7 million by mail for a total of 42.03 million. 

Republican lawmakers have lodged technical objections to early voting in several states where elections could be close. In Pennsylvania, for example, the state supreme court has ruled that mail-in votes cannot be counted if they do not bear a written date on the envelope -- a decision that could potentially affect thousands of ballots.

In Wisconsin, absentee votes can be thrown out if the address of the witness -- whom the state requires watch a ballot envelope being sealed -- is incomplete.

Former Republican president Donald Trump alleged last week that "unverified" ballots had been mailed to voters in the key state of Pennsylvania, a claim which was debunked by state elections officials.

After losing the 2020 presidential election to Democrat Joe Biden, Trump also made false claims of election fraud, which have been echoed by many members of his Republican Party.

Meanwhile, Trump traveled to Ohio to campaign alongside the Republican Senate nominee, J.D. Vance. He has repeatedly hinted that he plans to launch another presidential run soon. Despite delivering on campaign promises to boost infrastructure and clean energy, many Americans have soured on Biden's leadership. Only 39 percent approve of his job performance, according to a Reuters/Ipsos poll published on Monday.

Biden's unpopularity has made him an unwelcome guest in the most competitive races. On Monday, he spoke in reliably Democratic Maryland, where the Democratic gubernatorial nominee, Wes Moore, is widely expected to win back the Republican-held governorship.

If Republicans win the House or the Senate, that would spell the end of Biden's efforts to get abortion protections, family leave benefits and other Democratic priorities through Congress. It also would open the door for two years of Republican-led investigations that could potentially damage the White House. A Republican-led Senate could also block Biden's nominations for judicial or administrative posts.

Billionaire Elon Musk, whose purchase of Twitter and vows to loosen the reins on who can say what on the platform has led to some speculation that it could unleash a wave of disinformation, tweeted on Monday that "independent-minded voters" should vote for a Republican Congress "because shared power curbs the worst excesses of both parties."

If Republicans secure a House majority, they plan to use the federal debt ceiling as leverage to demand deep spending cuts. They would also seek to make Trump's 2017 individual tax cuts permanent and protect corporate tax cuts that Democrats have unsuccessfully tried to reverse over the past two years.

Control of Capitol Hill would give Republicans the power to block aid to Ukraine, but they are more likely to slow or pare back the flow of weapons and economic assistance to Kyiv than stop it.

When will the result be out

The first wave of vote tallies are expected on the East Coast between 7 p.m. and 8 p.m. ET (0000-0100 GMT Wednesday, Nov. 9). An early indication of Republican success could come if the races expected to be close - like Virginia's 7th congressional district or a U.S. Senate seat in North Carolina - turn out to be Democratic routs.

By around 10 p.m. or 11 p.m. ET, when polls in the Midwest will be closed for an hour or more, it's possible Republicans will have enough momentum for experts at U.S. media organizations to project control of the House, said Kyle Kondik, a political analyst at the University of Virginia's Center for Politics.

If the fight for the House still looks close as vote tallies start coming in from the West Coast - where there could be more than a dozen tight House races - it could be days before control of the chamber is known, experts said.

California typically takes weeks to count all its ballots, in part because it counts ballots postmarked by Election Day even if they arrive days afterward. Nevada and Washington state also allow late ballots if postmarked by Nov. 8, slowing down the march to final results.

"If the House is really on the edge, that would matter," said Kondik.

It may take longer, perhaps weeks longer, to know which party will control the Senate, with close contests in Pennsylvania, Arizona and Georgia likely to determine final control.

If Georgia's Senate race is as close as expected and no candidate receives more than 50% of the vote, a run-off election would be scheduled for Dec. 6, possibly leaving control of the chamber in limbo until then.