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, Huma Abedin believed to have been found on the laptop of her estranged husband, former congressman Anthony Weiner.
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.