WASHINGTON — Attorney General William P. Barr, wading into President Trump’s unfounded accusations of widespread election irregularities, told federal prosecutors on Monday that they were allowed to investigate “specific allegations” of voter fraud before the results of the presidential race are certified.
Mr. Barr’s authorization prompted the Justice Department official who oversees investigations of voter fraud, Richard Pilger, to step down from the post within hours, according to an email Mr. Pilger sent to colleagues that was obtained by The New York Times.
Mr. Barr said he had authorized “specific instances” of investigative steps in some cases. He made clear in a carefully worded memo that prosecutors had the authority to investigate, but he warned that “specious, speculative, fanciful or far-fetched claims should not be a basis for initiating federal inquiries.”
Mr. Barr’s directive ignored the Justice Department’s longstanding policies intended to keep law enforcement from affecting the outcome of an election. And it followed a move weeks before the election in which the department lifted a prohibition on voter fraud investigations before an election.
“Given that voting in our current elections has now concluded, I authorize you to pursue substantial allegations of voting and vote tabulation irregularities prior to the certification of elections in your jurisdictions,” Mr. Barr wrote.
A Justice Department official said that Mr. Barr had authorized scrutiny of allegations about ineligible voters in Nevada and backdated mail-in ballots in Pennsylvania. Republicans have circulated both claims in recent days without any evidence emerging to back them.
Mr. Barr did not write the memo at the direction of Mr. Trump, the White House or any Republican lawmakers, the official said.
Mr. Barr has privately told department officials in the days since the election that any disputes should be resolved in court by the campaigns themselves, according to three people briefed on the conversations. He has said that he did not see massive fraud, and that most of the allegations of voter fraud were related to individual instances that did not point to a larger systemic problem, the people said.
But critics of Mr. Barr immediately condemned the memo as a political act that undermined the Justice Department’s typical independence from the White House.
“It would be problematic enough if Barr were reversing longstanding Justice Department guidance because of significant, substantiated claims of misconduct — that could presumably be handled at the local and state level,” said Stephen I. Vladeck, a professor at the University of Texas School of Law.
“But to do so when there is no such evidence — and when the president’s clear strategy is to delegitimize the results of a proper election — is one of the more problematic acts of any attorney general in my lifetime,” Mr. Vladeck added.
Mr. Pilger, a career prosecutor in the department’s Public Integrity Section who oversaw voting-fraud-related investigations, told colleagues he would move to a nonsupervisory role working on corruption prosecutions.
“Having familiarized myself with the new policy and its ramifications,” he wrote, “I must regretfully resign from my role as director of the Election Crimes Branch.” A Justice Department spokeswoman did not immediately respond to a request for comment about Mr. Pilger’s message.
Justice Department policies prohibit federal prosecutors from taking overt steps, like questioning witnesses or securing subpoenas for documents, to open a criminal investigation into any election-related matter until after voting results have been certified to keep their existence from spilling into public view and influencing either voters or local election officials who ensure the integrity of the results.
“Public knowledge of a criminal investigation could impact the adjudication of election litigation and contests in state courts,” the Justice Department’s longstanding election guidelines for prosecutors say. “Accordingly, it is the general policy of the department not to conduct overt investigations.”
More covert investigative steps, like an investigator going undercover, are allowed but require the permission of a career prosecutor in the department’s Criminal Division.
Mr. Barr’s memo allows U.S. attorneys to bypass that career prosecutor and take their requests to his office for approval, effectively weakening a key safeguard that prevents political interference in an election by the party in power.
The memo is unlikely to change the outcome of the election but could damage public confidence in the results, Justice Department prosecutors warned, speaking on the condition of anonymity for fear of retribution. They said that the public posturing by the department also gave Republicans, including Senator Mitch McConnell of Kentucky, the majority leader, a tool to refuse to acknowledge Mr. Biden as the president-elect.
Mr. McConnell and Mr. Barr met on Monday afternoon. Representatives from both of their offices declined to comment on what they discussed.
Mr. Trump faces a steep battle in his attempt to change the election results. Mr. Biden declared victory on Saturday after several news media organizations declared him the winner based on tabulated election returns.
“It’s not merely about showing evidence of fraud but that the malfeasance would actually affect the outcome in several states,” said Matt Gorman, a Republican strategist. “You’re talking about changing hundreds of thousands of votes.”
While Mr. Trump’s campaign lawyers have filed a dozen or so legal challenges to the results in battleground states, none appeared to be gaining traction in the courts. And none were likely to give the president an edge in the votes he would need to change the outcome of the race.
Justice Department investigators are looking into a referral from the Republican Party in Nevada, which claims over 3,000 people who live outside the state voted in its election, the department official said. The official would not say whether the department had opened a full investigation. A federal judge dismissed the claim in court last week.
The department is also reviewing a sworn affidavit written by a postal worker in Erie, Pa., alleging that post office officials devised a plan to backdate mail ballots in the state, the official said.
The local postmaster has denied the allegations and said that the accuser has been disciplined multiple times in the past. That affidavit was sent to the department by Senator Lindsey Graham, Republican of South Carolina and head of the Senate Judiciary Committee, who is a close ally of the president.
In the days after the election, Mr. Barr faced pressure from Mr. Trump and his aides to intervene to help the president. Conservative commentators have criticized Mr. Barr’s lack of action, saying that he was looking the other way.
Mr. Barr had been silent about voter fraud in recent weeks after previously issuing unsubstantiated warnings of widespread fraud because of the large number of mail-in ballots cast in this election. Voter fraud is rare, and no major instances of it have emerged in the election.
At the same time, the department has made it easier for prosecutors to pursue voter fraud cases and publicized details from the investigations that generated headlines that helped Mr. Trump, prompting sharp criticism from Democrats and civil rights advocates.
Maggie Haberman contributed reporting.