Biden's team has made contact with two of most prominent leaders on the White House coronavirus task force -- Dr. Anthony Fauci, the nation’s top expert on infectious disease, and Dr. Deborah Birx, coronavirus response coordinator, as his transition team maintains its focus on the ongoing pandemic and the federal response they will inherit in 50 days.

Biden has held separate conversations with the two White House coronavirus task force officials, according to a transition official. Birx, whose colleagues told ABC News she would like to continue serving in a Biden administration, met with members of Biden's team on Monday.

A global health expert who was tapped from running the federal government’s program combating HIV/AIDS to serve on the task force in the February, Birx told CBS on Sunday that she was preparing to brief Biden's team on the nation's coronavirus response.

In an interview with McClatchy last week, Fauci said he assumes he will stay in his position as director of the National Institute of Allergy and Infectious Diseases, a post he has held for 36 years, but that he would also "seriously consider" serving in another capacity if Biden asked.

"I’m perfectly comfortable with the role that I’m in, but certainly if the president of the United States wants me to do something else, I’d seriously consider it," Fauci said. "Quite frankly, I don’t anticipate that I’m going to be doing anything other than what I’m doing now. But then again, we have a president-elect who may have other plans. I don’t know."

Ahead of Election Day, Biden promised he would "hire Fauci" and "fire Trump" if elected, amid Trump knocking Fauci's credibility and public health guidance.

-ABC News’ Benjamin Siegel and Ben Gittleson

Trump’s reelection campaign filed a petition to the Wisconsin Supreme Court on Tuesday, challenging the outcome of the presidential election that was certified on Monday with the president’s personal attorney, Rudy Giuliani, among those filing.

The legal action comes after the Trump campaign demanded recounts in Milwaukee and Dane counties, only to see that effort turn up 87 additional votes for his opponent, Biden. It also comes as Attorney General William Barr said in an [interview] () Tuesday that the Justice Department has uncovered no evidence of widespread voter fraud that would tip election results.

This latest petition alleges that poll workers illegally altered ballot envelopes, added missing information on voters’ behalf, counted ballots submitted early without sufficient voter identification, or collected ballots at unauthorized locations. In all, the Trump campaign claims that more than 211,000 votes were improperly counted. Trump lost the state to Biden by more than 20,600 votes.

Normally, this lawsuit would be filed with a circuit court but given the urgency the Trump campaign directly petitioned the state’s conservative-leaning high court. The court has not indicated if it will hear the case, or respond before the Dec. 8 “safe harbor” deadline, after which the election results are considered conclusive.

Democrats have not responded to the petition, but broadly have characterized the Trump campaign’s legal efforts as a “sideshow” that show no prospect of changing the outcome of the 2020 election. A series of court rulings to date have found no evidence of fraud or improper actions by elections officials in the states where the campaign has brought legal action.

-ABC News' Soo Rin Kim and Cheyenne Haslett

Attorney General William Barr told the Associated Press in an interview Tuesday that the Justice Department has not uncovered widespread fraud in the 2020 presidential election.

"To date, we have not seen fraud on a scale that could have affected a different outcome in the election," Barr told the AP.

The attorney general’s comments come into contrast with claims from the president and his lawyers that the election was stolen. Trump has still refused to concede to Biden.

"Most claims of fraud are very particularized to a particular set of circumstances or actors or conduct. They are not systemic allegations and. And those have been run down; they are being run down," Barr said.

"Some have been broad and potentially cover a few thousand votes. They have been followed up on."

Biden’s nominee to lead the Office of Budget and Management, Neera Tanden, would be the first woman of color and first South Asian American in the role if confirmed, but she is already facing resistance from some Senate Republicans who have signaled her nomination wouldn’t pass the Senate’s current GOP majority.

Tanden did not directly fire back against those attacks in remarks Tuesday but shared a personal story behind why she believes she’s in the position to fulfill the critical economic role which has sometimes served as a check within the executive branch on any far-fetched spending plans fancied by other Cabinet members.

"Like the vice president-elect’s mother, my mother, Mamala, was born in India. Like so many millions suppressed every generation she came to America to pursue a better life," Tanden said, going on to detail how after her parent’s divorce, their family relief on food stamps and public housing to survive. "We relied on a safety net to get back on her feet."

"I'm here today because of social programs, because of budgetary choices, because of a government that saw my mother's dignity and gave her a chance. Now it is my profound honor to help shape those budgets and programs to keep lifting Americans up," she said.

Tanden is currently the president and CEO of Center for American Progress, a center-left think tank, and a longtime adviser to Hillary Clinton.