US Senate Minority Leader Harry Reid accused FBI Director James Comey of breaking federal law in disclosing possible new evidence in the Hillary Clinton email investigation.
Reid said in a letter sent to Comey that his disclosure to Congress, made 11 days before the election, might have violated the Hatch Act, which prohibits partisan politicking by government employees.
"Your actions in recent months have demonstrated a disturbing double standard for the treatment of sensitive information, with what appears to be a clear intent to aid one political party over another," Reid wrote.
"I am writing to inform you that my office has determined that these actions may violate the Hatch Act, which bars FBI officials from using their official authority to influence an election. Through your partisan actions, you may have broken the law."
While many Democrats and some Republicans have cast doubt on Comey's actions, citing Justice Department policies and precedent on handling investigations ahead of elections, Reid's letter is the most forceful denunciation levelled by a high-ranking elected official.
In the letter, Reid drew a contrast between how Comey has treated the Clinton email probe and how he has handled what Reid described as "explosive information about close ties and coordination between Donald Trump, his top advisors, and the Russian Government."
"The public has a right to know this information," Reid said. "I wrote to you months ago calling for this information to be released to the public. There is no danger to American interests from releasing it. And yet, you continue to resist calls to inform the public of this critical information. By contrast, as soon as you came into possession of the slightest innuendo related to Secretary Clinton, you rushed to publicise it in the most negative light possible."
The FBI did not immediately respond to an email seeking comment on Reid's letter.
Republican Senator Tom Cotton of Arkansas denounced Reid from his Twitter account shortly after the letter was released.
Georgetown University law professor David Vladeck, who is critical of Comey's handling of the Clinton investigation, said that he did not believe that Comey had violated the Hatch Act.
"I do not think he has committed a crime," he said. But, he said, "I do think he has abused his office."
"Prosecutors have no warrant to characterise the behaviour of someone not charged with a crime," Vladeck said. "And it is grossly inappropriate for a prosecutor to fan political flames when there is no basis to even suggest any unlawful conduct."
Current and former officials could not understand how Comey could send the letter without knowing if, for instance, any of the emails were marked "classified" or if they were duplicates of those already in the FBI's possession.
In closing the letter, Reid told Comey that he has "been a supporter of yours in the past".
"When Republicans filibustered your nomination and delayed your confirmation longer than any previous nominee to your position, I led the fight to get you confirmed because I believed you to be a principled public servant," he said.
"With the deepest regret, I now see that I was wrong."