GOP lawmakers offered varying degrees of approval for Biden to begin receiving security briefings, with several Trump-aligned senators suggesting that they should begin as a precaution pending official certification of the vote, which could take weeks.
Others, including Sen. Susan Collins of Maine, advocated more forcefully for the administration to move on with the transition and stop blocking the briefings.
"President-elect Biden should be receiving intelligence briefings right now,” Collins said. “That is really important. It is probably the most important part of the transition … because we want the president-elect, assuming he prevails, to be ready on Day One."
Despite substantial vote margins in Biden’s favor across multiple swing states, the little-known General Services Administration has yet to officially "ascertain" an electoral winner, leaving the former vice president without access to crucial resources meant to ensure a smooth transition – including classified intelligence briefings.
Former officials said the administration not providing them could pose a risk national security – a claim echoed this week by Sen. John Thune, R-S.D., who said he believes Biden should have access to classified intelligence.
"I think it's important from a national security standpoint – continuity," Thune said.
The Republican chair of the Senate Intelligence Committee, Sen. Marco Rubio of Florida expressed support for Biden receiving briefings, but hedged his answer to align with the president’s efforts to overturn the election with a scattershot of legal challenges.
"I don’t think it prejudices any of the president’s claims in court or otherwise to ensure that every contingency is being accounted for," Rubio said. "When it comes to national security, we should do everything possible to ensure that, whatever the outcome for the next president, whether it’s the current one or Vice President Biden, are fully prepared."
Experts warned that foreign adversaries often seek to take advantage of American vulnerabilities during the transition period, making the continuity of information a critical element for new administrations.
"It is absolutely crucial that the Biden transition has the ability to think, plan, and prepare, because turning the ship of state is not something that happens quickly," said Dr. Christopher Swift, a national security lawyer and partner at Foley & Lardner LLP.
"If the Biden team wants to adapt to emerging challenges or be in a position to respond to immediate crises," Swift continued, "they need to understand what’s happening -- and where the pieces are on the global chess board."
After the contested election in 2000, for example, the 9/11 Commission Report determined that the compressed transition timeframe ahead of George W. Bush's inauguration "hampered the new administration in identifying, recruiting, clearing, and obtaining Senate confirmation of key appointees" – and ultimately impaired the government’s ability to respond to the terrorist attacks.
The Office of Director of National Intelligence said this week that it "would not have contact" with Biden's transition team or grant the president-elect briefings until the GSA officially declares him the winner.
But experts said the intelligence community is not beholden to the GSA "ascertainment." David Priess, a former intelligence briefer and author of "The President's Book of Secrets: The Untold Story of Intelligence Briefings to America's Presidents," said briefings could begin “independent of the GSA decision” at the direction of the Director of National intelligence or, of course, the president.
"The Presidential Transition Act of 1963 and its updates do not mention the President’s Daily Brief (PDB) or intelligence briefings for the president-elect," Priess said. “The custom of allowing the president-elect to see the PDB is just that— a custom — and it’s something the president can do, or refuse to do, at his discretion."
On Thursday, another notable voice in support of briefing Biden is Sen. Lindsey Graham, R-S.C., one of the president’s staunchest allies in the Senate.
Others Republicans joining the chorus include Mississippi Sen. Roger Wicker, South Dakota Sen. Mike Rounds, Texas Sens. John Cornyn and Ted Cruz, Missouri Sen. Roy Bunt, and Wisconsin Sen. Ron Johnson.
Sen. Josh Hawley, R-Mo., suggested Biden should be granted the candidate-style briefings he and Vice President-elect Kamala Harris received prior to the election, which amount to a watered-down source of major geopolitical developments – not the deeply classified President’s Daily Brief, to which a president-elect is typically entitled. It was not clear whether Biden has continued receive those candidate-style briefings.
Late Wednesday, Sen. James Lankford, R-Okla., pledged to “step in” on Friday if Biden hadn’t been granted access to the intelligence, later clarifying that it he would “reach out” to GSA to ensure they are preparing for a smooth transition of power. By Thursday, Lankford walked back his comments – noting that he only meant Biden should be offered the same briefings he received as a candidate.
Elsewhere in Washington, Trump-allied members of Congress and aides close to the president maintained Thursday that these briefings were "not necessary," as House Republican Leader Kevin McCarthy said Thursday.
"One president at a time," McCarthy said. "[Biden] is not president right now. I don't know if he will be president Jan. 20, but whoever is will get the information."
During a bizarre interview on Fox News Thursday morning, White House press secretary Kayleigh McEnany – appearing on the program in her capacity as an adviser to the Trump campaign – referred a question on the matter back to the White House.
"I haven't spoken to the president about that," McEnany said. “That would be a question more for the White House, but I will say that all laws are being followed with regard to an expected transition though we expect to continue on as the Trump administration, and we will see how our litigation goes."
For its part, the Biden camp seems unperturbed by transition delays. On Tuesday, the president-elect said his transition team is forging ahead with plans to take power in January, calling Trump's refusal to concede an "embarrassment."
ABC News' Allison Pecorin, Mariam Khan, and Sarah Kolinovsky contributed to this report.