Senator Patrick Leahy is among a group of Democrats concerned about a set of proposed rules designed to help the FBI catch terrorists.
The so-called attorney general guidelines have not been approved or even publicly released.
The rules are intended to update policies governing investigations as the FBI shifts from a traditional crime-fighting agency to one whose top priority is protecting the United States from terrorist attacks.
Leahy is the chairman of the Senate Judiciary Committee. He wants to guidelines delayed.
Others fear the rules could allow Americans to be targeted based on their race, ethnicity, religion – or free speech activities.
The Justice Department will review the request.