She suggested that if the program had been in place in 2001, it could have prevented the Sept. 11 terrorist attacks. She explained that in 2001, intelligence officials had vague fears of an impeding attack but did not have enough information to do anything about it.
"That can never be allowed to happen in the United States of America again," Feinstein said.
She acknowledged that leaks about the extent of the NSA's surveillance activities have eroded public trust in the agency, and she said some reforms are needed.
She is preparing her own legislation that would require more transparency about the NSA's activities but would preserve the NSA's bulk collection of phone data.
The program collects records, such as phone numbers, call times and call durations, but not the contents of the conversations.
But Leahy argued that proposals like Feinstein's don't go far enough to protect Americans' privacy.
"Additional transparency and oversight are important," Leahy said. "But I believe we have to do more."
Sen. Mike Lee (R-Utah) echoed Leahy's call to rein in the NSA's phone data collection.
“The Fourth Amendment’s protection against ‘unreasonable searches and seizures’ is designed to prevent against precisely the kind of suspicionless fishing expeditions that the government is currently carrying out under its telephony metadata collection program," Lee said.
But Sen. Jeff Sessions (R-Ala.) appeared to side with Feinstein, arguing that people have little privacy interest in their phone records.
"The records are in the possession of the phone company. They're the phone companies' records — they're not your personal records," Sessions said.
Sen. Lindsey Graham (R-S.C.) also expressed concern that ending the program would be a return to a "pre-9/11 mentality."