More than 40 million Americans have cast early ballots ahead of Tuesday's midterm elections, surpassing the numbers from two years ago, the US Elections Project said Monday.

Americans will go to the polls on Tuesday to elect 435 members of the House of Representatives, one-third of the Senate, and a host of state and local posts.

Most of the 50 US states allow voters to cast ballots early, either in-person or by mail, a practice that 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 18.5 early votes cast in-person and 22.3 million by mail for a total of 40.8 million.

It said 39.1 million people had voted by the same point in the 2018 midterms.

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 -- who the state requires watch a ballot envelope 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.