The Democratic leader in the US Senate says the head of the FBI may have broken the law by revealing the bureau was investigating emails possibly linked to Hillary Clinton.
Harry Reid accused FBI director James Comey of violating an act which bars officials from influencing an election.
News of the FBI inquiry comes less than two weeks before the US election.
The bureau has meanwhile obtained a warrant to search a cache of emails belonging to a top Clinton aide.
Emails from Huma Abedin are believed to have been found on the laptop of her estranged husband, former congressman Anthony Weiner.
There are reportedly 650,000 emails to search through, making it unlikely investigators can give a verdict on them before election day.
The FBI believes the emails might be “pertinent” to its previous inquiry into Mrs Clinton’s use of a private server when she was secretary of state in the Obama administration.
The case was closed in July without any charges being brought against Mrs Clinton.
Mr Weiner is subject to a separate investigation on suspicion of sending sexually explicit messages to an underage girl.
In a letter, Mr Reid accused Mr Comey of practising double standards with the intention of helping one political party over another.
He said Mr Comey may have violated the Hatch Act, which bars officials from using their position to influence an election.
“Through your partisan actions, you may have broken the law,” he said.
Mr Reid also accused Mr Comey of withholding “explosive information about close ties between [Republican candidate] Donald Trump, his top advisers, and the Russian government”.
“The public has a right to know about this information. I wrote to you months ago calling for this information to be released to the public,” Mr Reid said.
Richard Painter, a professor at the University of Minnesota Law School and the chief White House ethics lawyer from 2005 to 2007, revealed on Sunday he had filed a complaint against the FBI with the Office of Special Counsel, which investigates Hatch Act violations.
Writing in the New York Times he said: “I never thought that the FBI could be dragged into a political circus surrounding one of its investigations. Until this week.”
Race gets closer
With the election due on 8 November, opinion polls showed Mrs Clinton’s lead against Mr Trump tightening even before the email controversy surfaced again.
An ABC News/Washington Post poll published on Sunday put Mrs Clinton just one percentage point ahead.
Mrs Clinton has described Mr Comey’s actions as “unprecedented” and “deeply troubling”.
But Mr Trump has praised the FBI’s decision, accusing the Department of Justice of protecting Mrs Clinton in a “rigged system”.
“The Department of Justice is trying their hardest to protect the criminal activity of Hillary Clinton,” Mr Trump told a rally in Nevada.
It emerged on Sunday that the Department of Justice had urged the FBI not to inform Congress of the new inquiry so close to the election.