Americans will go to the polls on Tuesday, November 8, to elect all 435 members of the House of Representatives, one-third of the Senate and a host of state and local posts.
According to the U.S. Elections Project, as of Monday, there have been more than 19.3 million early votes cast in-person and 22.7 million by mail for a total of 42.03 million.
It said 39.1 million people had voted by the same point in the 2018 midterms.
Most of the 50 U.S. 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.
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.
Speaking about the election process to CNN's "State of the Union" show, Ronna McDaniel, Republican Party chair, said Republicans "want to make sure it's run fair and transparently and then we'll let the process play out, and then we'll accept the results."
Polls in the final stretch put Republicans ahead in the fight for the House of Representatives, and also show them gaining momentum in key Senate races as voters seek to take out frustration over four-decades-high inflation and rising illegal immigration.