WASHINGTON (AP) -- The Trump administration threw the presidential transition into tumult on Monday, with President Donald Trump blocking government officials from cooperating with President-elect Joe Biden's team and Attorney General William Barr authorizing the Justice Department to probe unsubstantiated allegations of voter fraud.

Some Republicans, including Senate Majority Leader Mitch McConnell, rallied behind Trump's efforts to fight the election results. Few in the GOP acknowledged Biden's victory or condemned Trump's other concerning move on Monday: his firing of Defense Secretary Mark Esper.

The developments cast doubt on whether the nation would witness the same kind of smooth transition of power that has long anchored its democracy. The Electoral College is slated to formally confirm Biden's victory on Dec. 14 and the Democrat will be sworn into office in late January.

On Monday, Barr authorized US attorneys to probe "substantial" allegations of voter irregularities and election fraud, though no widespread instances of that type of trouble in the 2020 election exist. In fact, election officials from both political parties have publicly stated that voting went well and international observers also confirmed that there were no serious irregularities.

Biden campaign lawyer Bob Bauer said Barr's memorandum authorizing investigations "will only fuel the 'specious, speculative, fanciful or far-fetched claims' he professes to guard against."

Biden pressed forward with plans to build out his administration, assembling a team of experts to face the surging pandemic. But the federal agency that needs to greenlight the beginnings of the transition of power held off on taking that step. And the White House moved to crack down on those not deemed sufficiently loyal as Trump continued to refuse to concede the race.

Trump remained out of sight at the White House, with conversations ongoing about how the defeated president would spend the coming days and weeks as he challenged the people's verdict. Trump is not expected to formally concede but is likely to grudgingly vacate the White House at the end of his term, according to several people around him.

Also being discussed: the possibility of more campaign-style rallies as he tries to keep his supporters fired up despite his defeat. It was possible they would feature his family and top supporters but not the president himself.

The ouster of Esper, the Pentagon chief, was expected by some aides to be the first of several firings by Trump, now freed from having to face voters again and angry at those in his administration perceived to be insufficiently loyal. Others believed to be vulnerable: FBI Director Christopher Wray, CIA head Gina Haspel and infectious disease expert Dr. Anthony Fauci.

The president was given cover to keep fighting by McConnell, seen by many in the GOP as the one who may eventually need to nudge Trump to the exit.

"Our institutions are actually built for this," McConnell said as he opened the Senate on Monday. "We have the system in place to consider concerns and President Trump is 100 percent within his rights to look into allegations of irregularities and weigh his legal options."

Senate Democratic leader Chuck Schumer countered that the Republicans' refusal to accept the election results was "extremely dangerous, extremely poisonous to our democracy."

"Joe Biden won the election fair and square," Schumer said.

A few other GOP senators sent tepid nods toward a transition. Sen. Ben Sasse of Nebraska offered congratulations to Biden, and Sen. Susan Collins of Maine noted the Democrat's "apparent victory." But many Republican lawmakers were reluctant to speak up about the election, seeing little political incentive to take a firm stance on Trump's transition from the White House.

Republicans on Capitol Hill have been hesitant to push Trump to concede to Biden, knowing it would anger their base of Trump's most devoted supporters. Most were also not overtly encouraging the president's unfounded claims of fraud, while allowing baseless questions about the election process to linger.

Adding to the sense of uncertainty, the General Services Administration held off on formally beginning the transition, preventing Biden's teams from gaining access to federal agencies. An agency spokesperson said late Monday that an "ascertainment" on the winner of the election had not yet been made. Citing what the agency did during the extended 2000 electoral recount, it signaled that it may not do so until Trump concedes or the Electoral College meets next month.

That Florida recount involved a margin of just 537 votes in the one state that would have determined which candidate reached 270 electoral votes. Biden's leads across Wisconsin, Pennsylvania and Michigan, which pushed him over the threshold to win the White House, are far more substantial -- and greater than Trump's leads in the same states in 2016.

On a call Monday night with reporters, a transition official said the Biden team believes it is time for the GSA administrator to ascertain that Biden is president-elect. The official, who spoke only on anonymity as a ground rule for the call, said legal action is "certainly a possibility" if that doesn't happen, though there are also other options being considered.

Across government, there were signs of a slowdown.

White House officials and Trump political appointees informed career government staffers they were not to begin acting on transition planning until GSA approved it, according to officials familiar with the matter.

In weekly Monday morning all-hands phone calls for Midwest-based employees of the Environmental Protection Agency, mid-level administrators responded to questions about the transition by telling staffers they had no information yet, said Nicole Cantello, an agency employee and president of the Chicago local of a union representing EPA workers.

A senior administration official said presidential personnel director John McEntee, the president's former personal aide, has sent word to departments that they should terminate any political appointees seeking new work for now. Another official said the warning was not seen as likely to result in any firings but rather meant to reinforce to staff that they should not act counter to Trump while he refuses to concede. Those officials and others who were not authorized to discuss internal policies or describe private discussions requested anonymity.

But some elements of the federal government already were mobilizing to prepare for Biden to assume power. The US Secret Service and Federal Aviation Administration extended a flight restriction over Biden's Wilmington, Delaware, home through Inauguration Day. Biden's security detail has been bolstered with agents from the Presidential Protective Division.

And despite Trump's public stance, there was a growing realization in his inner circle that the election result would be impossible to overturn. Some senior officials have tried to make the case that Trump should turn his efforts to cementing his legacy, but they are wary of being labeled disloyal for even thinking it.

Legal challenges already have been dismissed in battleground states like Georgia and Wisconsin. And Trump's legal efforts were dealt another blow Monday when campaign adviser David Bossie, tasked with leading the effort, tested positive for the coronavirus.

Bossie had been at the indoor White House election night party now being perceived as a possible superspreader event after other attendees -- including chief of staff Mark Meadows, Secretary of Housing and Urban Development Ben Carson and other aides -- contracted the virus.

At the White House, attendance among aides had dropped off since election night -- partly because of the result and partly because a number are in quarantine after contracting or being exposed to people who came down with COVID-19. Vice President Mike Pence was slated to depart Tuesday for a vacation in Florida after a visit to Capitol Hill.

Trump's public schedule hasn't included an intelligence briefing since Oct. 1. The White House hasn't provided a "readout" of any call between the president and a foreign leader in weeks. He hasn't met with members of the White House coronavirus task force in months. He also offered no public comment on Tropical Storm Eta lashing the Florida Keys.

The drawn-out resolution to the election has only added to the culture of suspicion that has permeated the hollowed-out West Wing.