Specter, who will hold hearings next month on the decision to allow the NSA program without court approval, said he has told Bush administration officials he believes they are on shaky legal ground.

Bush has pointed to a congressional resolution passed after the attacks of Sept. 11, 2001, that authorized him to use force in the fight against terrorism as allowing him to order the program. The program authorized eavesdropping of international phone calls and e-mails of people deemed a terror risk.

"I thought they were wrong," Specter said on ABC's "This Week." "There still may be different collateral powers under wartime situations. That is a knotty question."

A number of members of Specter's committee, including GOP Sen. Sam Brownback of Kansas, have expressed doubt about the administration's legal basis. The hearings, planned for early February, will feature Attorney General Alberto Gonzales.

Specter noted that impeachment and criminal prosecution are possibilities in the event a president acted unconstitutionally.

But Specter added: "I don't see any talk about impeachment here. I don't think anyone doubts the president is making a good-faith effort. He's acting in a way that he feels he must."