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

Senate Majority Leader Mitch McConnell, the top Republican in Congress, acknowledged Biden’s win for the first time from the Senate floor Tuesday morning after weeks of holding out. 

"Our country has officially a president-elect and a vice-president elect," McConnell said. "The Electoral College has spoken, so today I want to congratulate President-elect Joe Biden."

McConnell also congratulated Vice President-elect Kamala Harris on historic nature of her win.

His acknowledgement comes one day after the Electoral College cemented Biden’s victory. Now that McConnell has accepted the incoming adminstration, it’s expected other Congressional Republicans will fall in line. 

Before McConnell took the floor, Senate Chaplain Barry Black opened Tuesday’s session by saying the Electoral College decision has been accepted and callings for blessings in his opening prayer on "President-elect Joe Biden and Vice President-elect Kamala Harris."

-ABC News' Trish Turner

Fresh off the Electoral College affirming his win, Biden is returning to the campaign trail Tuesday to stump for Georgia's Senate runoffs and the promise he could pass the big ticket legislation through Congress he's hoping for -- as the balance of power in the incoming Senate hangs on two seats there.

Biden is slated to speak at an afternoon, drive-in rally in Atlanta for Democratic Jon Ossoff and Rev. Raphael Warnock, challengers to sitting GOP Sens. Kelly Loeffler and David Purdue in the Jan. 5 runoff. Early voting kicked off in Georgia Monday.

After the Electoral College officially cemented his victory Monday, Biden delivered his strongest rebuke of Trump and his ongoing challenges to the election’s outcome in a primetime address, calling on the country to move on and noting that Trump hasn't been able to prove the allegations of fraud he's made.

“The integrity of our elections remains intact. And now it is time to turn the page, as we’ve done throughout our history. To unite. To heal," Biden said.

The pressure is now on Senate Majority Leader Mitch McConnell to do just that as he's yet to publicly acknowledge Biden's win even as some Senate Republicans -- and Russian President Vladimir Putin -- have done so since the Electoral College vote.

Trump has no public events on his schedule Tuesday -- refusing to acknowledge both his election loss and the country's worsening death toll from the coronavirus pandemic -- but continues to air his grievances with the election and a litany of false claims on Twitter.

The president also announced on Twitter Monday Attorney General William Barr is resigning from the administration before Christmas -- with only a month left to go.

After weeks of holding out, Russian President Vladimir Putin has congratulated Biden on winning the presidential election following Monday's Electoral College voting affirming Biden's win.

According to the Kremlin in a statement released Tuesday, "Vladimir Putin wished the president-elect every success and expressed confidence that Russia and the United States, which bear special responsibility for global security and stability, can, despite their differences, effectively contribute to solving many problems and meeting challenges that the world is facing today."

"For my part, I am ready for interaction and contact with you," Putin added, according to the Kremlin.

While Putin has now congratulated the president-elect on his victory, Republican leadership in Congress still has not -- at least not publicly.

The nation's top infectious disease expert Dr. Anthony Fauci on ABC's "Good Morning America" Tuesday said that it's his "strong recommendation" that Biden and Vice President-elect Harris should receive the COVID-19 vaccine as soon as its available to them.

"For security reasons, I really feel strongly we should get them vaccinated as soon as we can and want him fully protected as he enters the presidency in January. That's my strong recommendation," Fauci said. 

When asked Monday about Biden’s plans to get the COVID-19 vaccine, a transition official told ABC News the president-elect is in touch with Fauci, and plans to take the vaccine in public when he recommends he do so. 

Fauci added that if Trump were to seek his advice on the vaccine, he would also recommend that he and Vice President Mike Pence receive the vaccine, even though the president already had COVID-19 and likely has some protective antibodies. 

"We're not sure how long that protection lasts, so to be doubly sure I would recommend that he get vaccinated as well as the vice president," he said. 

Fauci, who is advising the current administration and plans to serve as Biden's chief medical adviser on the pandemic, has said he thought he would get vaccinated himself publicly "within a week."

-ABC News' Katherine Faulders and Shannon Crawford