US attorney general William Barr has allowed prosecutors to probe alleged irregularities in the presidential election, prompting a senior justice department official to quit.
The official, Richard Pilger, would have overseen such investigations.
Any such cases would normally be the remit of individual states, but Mr Barr said this was not a hard and fast rule.
Donald Trump refuses to accept Joe Biden's projected victory, and has made unsubstantiated fraud claims.
The president's campaign is seeking an emergency injunction in Pennsylvania to prevent Mr Biden's victory being certified in the state.
The president-elect's projected win there on Saturday took him over the threshold of 270 electoral college votes needed to secure victory nationwide.
Mr Trump's spokeswoman vowed the legal battle to contest Mr Biden's victory was only just beginning.
"This election is not over," White House press secretary Kayleigh McEnany told a news conference. "Far from it."
What is Mr Barr saying?
The attorney general wrote that inquiries could be made by federal prosecutors "if there are clear and apparently-credible allegations of irregularities that, if true, could potentially impact the outcome of a federal election in an individual State".
Mr Barr said prosecutors should only look into "substantial allegations" of irregularities, and that "specious, speculative, fanciful or far-fetched claims" should be ignored.
He acknowledged that individual states had the primary responsibility for the conduct of elections but said the justice department had "an obligation to ensure that federal elections are conducted in such a way that the American people can have full confidence in their electoral process and their government".
The department would normally only go beyond preliminary investigations after an election had been concluded and the results certified, but Mr Barr said this could result in situations where "misconduct cannot realistically be rectified".
Mr Pilger said he had quit in response to Mr Barr's memo.
"Having familiarised myself with the new policy and its ramifications... I must regretfully resign from my role," he wrote in an email to colleagues.
Separately, the Trump campaign filed a lawsuit on Monday in a Pennsylvania federal court, seeking an emergency injunction to stop state officials from certifying Mr Biden's victory in the state. The state's Attorney General, Josh Shapiro, called the lawsuit "meritless".
Prosecutors in Republican-controlled states meanwhile threw their weight behind the president's challenge to the election results.
The 10 state attorneys general filed a so-called amicus brief at the US Supreme Court backing the Trump campaign's case in Pennsylvania.
What are Biden and Trump up to?
Since media projected on Saturday that Mr Biden had won the critical state of Pennsylvania, accumulating enough votes to claim the White House, the president-elect has forged ahead with his plans to take the reins of power.
Mr Trump took to Twitter again on Monday to dispute the outcome, making unsubstantiated claims of "unthinkable and illegal" activity in the vote.
The General Services Administration (GSA), which manages federal agencies, has held off on allowing Biden aides to formally begin the transition, saying no "ascertainment" on an election winner had yet been made.
The GSA is tasked with formally recognising the president-elect, and providing the funds and access to federal agencies that Mr Biden's transition team needs.
In previous elections it has mostly acted promptly after the winner was declared, with the exception of the disputed election in 2000 when it waited until a Supreme Court ruling decided the result on 14 December.
CBS News, the BBC's US partner, says the Biden team is considering its legal options if the Trump administration continues to stall the handover.
White House reporters have been saying that despite his objections, Mr Trump is expected to leave office begrudgingly in January and is already talking about running for the White House again in 2024.
What is the Trump campaign alleging?
At Monday's news conference, Ms McEnany and Republican National Committee (RNC) chairwoman Ronna McDaniel cited allegations of electoral corruption, while urging reporters to help investigate the unverified claims.
Conservative channel Fox News cut away from the event, citing the lack of evidence.
"Whoa, whoa, whoa," said presenter Neil Cavuto. "Unless she [Ms McEnany] has more details to back that up, I can't in good countenance continue to show you this."
Ms McEnany told reporters: "We have only begun the process of obtaining an accurate, honest vote count."
She said Republican poll watchers had not been granted adequate access to vote counts in the city of Philadelphia, Pennsylvania.
According to the Philadelphia Inquirer, both Republican and Democratic election monitors were kept from 13ft to 100ft (4m to 30m) away from tables where votes were being tallied in the city, and local election officials cited coronavirus prevention needs for the distancing.
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