WASHINGTON - Several senior Republican U.S. senators on Monday acknowledged Democrat Joe Biden as the country's president-elect after the Electoral College affirmed his victory, and rejected the idea of overturning the 2020 presidential election in Congress.

As a few of President Donald Trump's most loyal backers contemplated a last-ditch effort to reverse his Nov. 3 election defeat, a growing number of Republicans appeared ready to defy Trump and recognize Biden as the winner more than a month after the vote.

Senator John Thune, the Senate's No. 2 Republican, said lawmakers had the right to challenge electoral votes. But he also said it was "time to move on" and that as soon as Biden "crosses the 270-vote threshold" in the Electoral College, he would be president-elect.

A candidate needs 270 Electoral College votes to win the White House. Biden passed that threshold on Monday afternoon when California delivered its 55 electoral votes to the Democrat.  He will take office on Jan. 20.

Other Republican senators who publicly recognized Biden as president-elect on Monday included Lindsey Graham, a Trump loyalist from South Carolina; Ohio's Rob Portman; Missouri's Roy Blunt, the Senate's No. 4 Republican; and Shelley Moore Capito of West Virginia.

Thune added that any effort to try to overturn the result when Congress counts the Electoral College votes on Jan. 6 would get little support. "It's not going anywhere," he told reporters.

Senator John Cornyn, another veteran Republican, said he thought any such effort "would be a bad mistake" that would be soundly defeated in the 100-member chamber, currently controlled by his party.

"There comes a time when you have to realize that, despite your best efforts, you've been unsuccessful, that’s sort of the nature of these elections. You've got to have a winner. You've got to have a loser," Cornyn told reporters at the U.S. Capitol.

He said he expected there would be a peaceful transition of power from Trump, who has so far refused to concede the election and launched dozens of unsuccessful lawsuits seeking to overturn his loss to Biden.

On Monday, Trump repeated a series of unsupported claims of electoral fraud.

Any effort to try to persuade Congress not to certify the Electoral College count on Jan. 6 must earn majority approval from both chambers of Congress that day. Democrats control the House of Representatives, while enough Republicans in the Senate have acknowledged Biden's victory to ensure any challenge would almost certainly fail. 

(Reporting by Patricia Zengerle and Tim Reid; Editing by Colleen Jenkins, Scott Malone and Peter Cooney) ((patricia.zengerle@thomsonreuters.com, www.twitter.com/ReutersZengerle; 001-202-898-8390;))