The rules, known as the attorney general guidelines, have not been approved or even publicly released yet, but four Democrats joined a growing chorus of lawmakers raising concerns after being briefed on what the guidelines say.
Among their fears: Americans could be targeted in part based on their race, ethnicity or religion - or free speech activities protected by the Constitution.
"As you know, attorney general guidelines were first implemented in the wake of the FBI abuses of the 1960s and 1970s, and serve as one of the most important bulwarks against future abuses," the senators said in a letter to Attorney General Michael Mukasey.
The four Democrats - Sens. Dick Durbin of Illinois, Russ Feingold of Wisconsin, Edward M. Kennedy of Massachusetts and Sheldon Whitehouse of Rhode Island - indicated they remained concerned even after assurances from officials during the Justice Department briefings.
The lawmakers asked Mukasey to hold off finalizing the rules to allow a public review.
"Given the importance of these guidelines, providing a period of time for public comment would be a reasonable and responsible way to move forward and achieve the best possible end result," the Democrats wrote.
Earlier this week, Senate Judiciary Chairman Patrick Leahy, D-Vt., and the panel's top Republican, Sen. Arlen Specter of Pennsylvania, also called for delaying the guidelines.
Justice spokesman Brian Roehrkasse said the department will review the requests. Citing remarks earlier by Mukasey about the new rules, the spokesman said an investigation would not be opened based solely on a person's race, ethnicity or religion.
"The guidelines will require all activities to have a valid purpose," Roehrkasse said, adding that the rules will "include robust and effective oversight measures."
The guidelines are expected to be finalized next week. They do not require congressional approval.
First reported last month by The Associated Press, 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.
Currently, the FBI must have evidence or allegations of wrongdoing before opening an investigation of U.S. citizens or legal residents from other countries. As described by some law enforcement officials, the new policy would let agents open preliminary terrorism investigations after mining public records and intelligence to build a profile of traits that, taken together, were deemed suspicious.
The officials, who spoke on condition of anonymity because they were not authorized to speak publicly about the rules, said factors that could trigger an inquiry would include travel to regions of the world known for terrorist activity and access to weapons or military training, along with the person's race or ethnicity.
Following their briefings, the four Democrats said the guidelines would:
Let the FBI use "a variety of intrusive investigative techniques" with no evidence of possible wrongdoing. The techniques could include: long-term FBI surveillance, interviewing neighbors and work-mates, recruiting informants and searching commercial databases for information on people "all without any basis for suspicion."
"We are particularly concerned that the draft guidelines might permit an innocent American to be subjected to such intrusive surveillance based in part on race, ethnicity, national origin, religion, or on protected First Amendment activities," the senators wrote.
Allow the government to collect foreign intelligence information inside the United States without current legal protections for U.S. citizens or legal residents. The senators noted that the broad term "foreign intelligence" would cover any information relating to the activities of a foreign government, organization or person.
Allow the information gathered to be broadly shared among government agencies. "We have serious questions about the scope of information sharing as it relates to U.S. persons who are under no suspicion of wrongdoing," the senators wrote.