Trump-Biden transition: Attorney Sidney Powell back at White House Sunday

Ahead of meeting virtually with a group of governors, Biden is expected to introduce former primary campaign rival Pete Buttigieg as his nominee for transportation secretary from Wilmington, Delaware, on Wednesday, as he continues to build out his incoming Cabinet. At age 38, Buttigieg would be Biden’s youngest nominee to date, and if confirmed he would become the first openly gay Cabinet member.

Biden also has other picks in the pipeline -- including the nomination of former Michigan Governor Jennifer Granholm to head the Department of Energy, according to sources, and the appointment of Gina McCarthy, who served as administrator of the Environmental Protection Agency in the Obama Administration, as his top domestic climate policy aide, according to a source familiar with the decision. 

Trump, meanwhile, is slated to convene his first Cabinet meeting since Election Day. The president has maintained a light public schedule since the election, turning to Twitter instead to air his grievances with the process. Most recently, he told Senate Majority Leader Mitch McConnell that it’s “too soon to give up” after McConnell acknowledged Biden as the president-elect for the first time on Tuesday.

Hours after recognizing Biden as the president-elect, McConnell warned Republicans on a conference call Tuesday that a Jan. 6 fight over electors would be a "terrible vote" and divisive for the party, sources familiar with the comments told ABC News. But some of Trump's Republican allies are still committed to challenging results on the House floor -- even as Senate Republicans have begun acknowledging Biden's victory. 

"I have a choice, I can either fight, or I can join the surrender caucus," Rep. Mo Brooks, R-Ala., one of the leaders of the effort to challenge the certification of the Electoral College, told ABC News. Brooks said Monday's Electoral College vote hasn't changed his plans, but he has yet to find a GOP senator to back him to challenge the vote (a representative from each chamber is necessary).

Brooks might find a willing senator on Capitol Hill Wednesday when Sen. Ron Johnson, R-Wis., chairman of the Homeland Security and Governmental Affairs Committee, holds a hearing on "election irregularities." The hearing comes despite the fact that the Cybersecurity and Infrastructure Security Agency deemed the 2020 election the most secure in history, and Attorney General William Barr, set to depart the White House before Christmas, said the Department of Justice found no evidence of widespread voter fraud that would change the results of the election.

President-elect Joe Biden is expected to name Gina McCarthy, who served as administrator of the Environmental Protection Agency under President Barack Obama, his top domestic climate policy aide, according to a source familiar with the decision. 

McCarthy, who currently serves as the president of the National Resources Defense Council, is expected to coordinate domestic efforts to slow climate change across the administration from the White House. 

It's the second climate-focused White House position created by Biden, following his decision to enlist former Secretary of State John Kerry to serve as a climate envoy, coordinating policy with other countries.  

In addition to her work in the Obama administration as EPA administrator from 2013-2017, McCarthy served as commissioner of the Connecticut Department of Environmental Protection, and in several similar posts in Massachusetts, where she advised several governors on environmental issues.

-ABC News' Benjamin Siegel and Molly Nagle

Some of President Trump's Republican allies are still committed to challenging the 2020 election results on the floor of the House of Representatives next month -- even as Senate Republicans have begun acknowledging Biden's status as the president-elect.  

"I have a choice -- I can either fight, or I can join the surrender caucus," Rep. Mo Brooks, R-Alabama, one of the leaders of the effort to challenge the certification of the Electoral College, told ABC News. 

Brooks said Monday's Electoral College vote hasn't changed his plans, but he has yet to find a GOP senator to back him. Without one, he can't force Congress to debate and vote on whether to accept a given state's slate of electors. 

"This is purely performative," Michael Waldman, president of the Brennan Center, told ABC News of the long-shot challenge, comparing it to the Texas Supreme Court lawsuit that unsuccessfully sought to overturn the results in several key states. 

Forcing senators on the record "purely creates political pain right now for Republicans," he said. 

Republican leaders in the Senate, wary of such a vote, have also privately discouraged members from signing on. 

Senate Majority Leader Mitch McConnell, soon after recognizing Biden as the president-elect, warned Republicans on a conference call that a Jan. 6 fight over electors would be a "terrible vote" and divisive for the party, sources familiar with the comments told ABC News. 

-ABC News' Benjamin Siegel and Trish Turner

Biden is expected to name former Michigan Gov. Jennifer Granholm to head the Department of Energy, sources familiar with the decision confirmed to ABC News on Tuesday.

That would make Granholm, 61, the ninth woman Biden has nominated so far to serve in his cabinet. If confirmed by the Senate, the former two-term governor would be just the second woman to lead the Energy Department.

A source familiar with the transition team's thinking pointed to Granholm’s work with Biden in Michigan during the 2009 recovery that rescued the U.S. auto industry and led to investments in green energy jobs in the state -- something Biden’s ambitious infrastructure plan would also seek to invest in.

-ABC News' John Verhovek, Molly Nagle and Benjamin Siegel