Harry Reid, the Democratic leader in the Senate, says FBI director James Comey may have broken the law by revealing the bureau was investigating emails possibly linked to Hillary Clinton.
He accused 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 Election Day.
The FBI has meanwhile obtained a warrant to search a cache of emails belonging to Huma Abedin, a top Hillary 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 case was closed in July without any charges being brought against Hillary Clinton.
Anthony Weiner is subject to a separate investigation on suspicion of sending explicit messages to an underage girl.
Harry Reid accused James Comey of practicing double standards with the intention of helping one political party over another.
In a letter, Harry Reid said James 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.
Harry Reid also accused James 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,” Harry 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 October 30 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.”
Opinion polls showed Hillary Clinton’s lead against Donald Trump tightening even before the email controversy surfaced again.
An ABC News/Washington Post poll published on October 30 put Hillary Clinton just one percentage point ahead.
Hillary Clinton has described James Comey’s actions as “unprecedented” and “deeply troubling”.
However, Donald Trump has praised the FBI’s decision, accusing the Department of Justice of protecting Hillary Clinton in a “rigged system”.
“The Department of Justice is trying their hardest to protect the criminal activity of Hillary Clinton,” Donald Trump told a rally in Nevada.
It emerged on October 30 that the Department of Justice had urged the FBI not to inform Congress of the new inquiry so close to the election.