With Biden set to be inaugurated on January 20, Trump is intensifying efforts to prevent the traditional transfer of power, ripping the party apart.

He's enlisted a dozen Republican senators and up to 100 House Republicans to challenge the Electoral College vote when Congress convenes in a joint session on Wednesday.

Despite Trump's claims of voter fraud, state officials have insisted the elections ran smoothly and there was no evidence of fraud or other problems that would change the outcome. The states have certified their results as fair and valid. Of the more than 50 lawsuits the president and his allies have filed challenging election results, nearly all have been dismissed or dropped. He's also lost twice at the US Supreme Court.

In audio disclosed on Sunday, Trump can be heard pressuring Georgia Secretary of State Brad Raffensperger to "find" him more votes.

"So look. All I want to do is this. I just want to find 11,780 votes, which is one more than we have. Because we won the state," Trump said in the recording, insisting there was "no way" he lost the state.

Raffensperger repeatedly countered the president's assertions.

Some prominent Republicans have now joined Democrats in pushing back.

"The 2020 election is over," said a statement from a bipartisan group of 10 senators, including Republicans Susan Collins, Lisa Murkowski, Bill Cassidy and Mitt Romney.

The senators wrote that further attempts to cast doubt on the election are "contrary to the clearly expressed will of the American people and only serve to undermine Americans' confidence in the already determined election results."

Republican Governor Larry Hogan of Maryland said, "The scheme by members of Congress to reject the certification of the presidential election makes a mockery of our system and who we are as Americans."

Former House Speaker Paul Ryan, a Republican, said in a statement that "Biden's victory is entirely legitimate" and that efforts to sow doubt about the election "strike at the foundation of our republic."

Liz Cheney, the third-ranking House Republican, warned in a memo to colleagues that objections to the Electoral College results "set an exceptionally dangerous precedent."

Other prominent former officials also criticised the ongoing attack on election results. In the Washington Post, 10 former defence secretaries -- half of them having served Republican presidents -- called on Pentagon officials to carry out the transition to the new administration "fully, cooperatively and transparently." They also asserted that efforts to involve the U.S. armed forces in resolving election disputes "would take us into dangerous, unlawful and unconstitutional territory."

Citing election results, legal challenges, state certifications and the Electoral College vote, the former defense secretaries said that "the time for questioning the results has passed; the time for the formal counting of the electoral college votes, as prescribed in the Constitution and statute, has arrived."

The moment is a defining one for the Republican Party in a post-Trump era.

Vice-president Mike Pence will be carefully watched as he presides over what is expected to be a prolonged showdown, depending on how many challenges are mounted.

The vice president "welcomes the efforts of members of the House and Senate to use the authority they have under the law to raise objections," Pence's chief of staff, Marc Short, said in a statement Saturday.

Australian Associated Press