A nonpartisan group of presidential transition experts is urging the General Services Administration (GSA) to certify President-elect Joe Biden (D) as the winner of the 2020 election and begin the presidential transition process.

In a letter from the Center for Presidential Transition, a nonpartisan group that advises incoming administrations, several former top officials, including former President George W. Bush's chief of staff, Josh Bolten, urged the Trump administration and Biden campaign to begin work on an orderly White House transition.

"We urge the Trump administration to immediately begin the post-election transition process and the Biden team to take full advantage of the resources available under the Presidential Transition Act. This was a hard-fought campaign, but history is replete with examples of presidents who emerged from such campaigns to graciously assist their successors," they wrote.

The group's message comes as President Trump has refused to concede defeat in the 2020 election and has vowed legal challenges in several states, though major news networks projected Saturday that he would be defeated as several states continue to count mail-in ballots.

A spokesperson for the agency indicated Sunday that the GSA would not move to begin the transition process until Biden's win was legally certified, though it wasn't clear which legal authority would make that certification.

“An ascertainment has not yet been made,” Pamela Pennington, the agency's spokesperson, told The Washington Post, “and its Administrator will continue to abide by, and fulfill, all requirements under the law.”

Trump's former White House chief of staff Mick Mulvaney said Thursday that he expected the president to submit to a peaceful transition of power should Biden be declared the winner, though it remains unclear when he will accept the election results.

"If the process runs, and I expect it to run, and at the end of that process Joe Biden's the president, you can absolutely guarantee a peaceful transition of power. I just hope the same is true on the other side," Mulvaney told CNBC.