Before Mr. Comey’s testimony, the White House had largely been able to fend off aggressive oversight of the N.S.A. wiretapping program since it was first disclosed in December 2005. The Republican-controlled Congress held a series of hearings last year, and even considered several legislative proposals to curb the scope of the eavesdropping. But Mr. Cheney repeatedly pressured Republican Congressional leaders to pull back without forcing the administration’s hand.

When the Democrats won the 2006 midterm elections, many political analysts predicted that the N.S.A. program — which a federal judge declared unconstitutional — would be one of the first Bush administration operations to undergo new scrutiny. But in January, administration officials announced that it was placing the program under the legal framework of the Foreign Intelligence Surveillance Act, a move it had previously refused to consider.

The Democrats have largely focused on objections to the war in Iraq in their first months in power, and had appeared reluctant to take aggressive steps to challenge policies on harsh interrogation practices, secret Central Intelligence Agency prisons and wiretapping without warrants, for fear of being labeled soft on terrorism.

For instance, at a confirmation hearing June 19 for John A. Rizzo as general counsel of the C.I.A., no member of the Senate Intelligence Committee directly challenged the agency’s practices of secret detention or harsh interrogation.

Mr. Rizzo successfully dodged the tougher questions by saying he preferred to answer them in closed session. The Senate Intelligence Committee has conducted closed-door oversight of the N.S.A. wiretapping program, but it has not been as aggressive as the Judiciary Committee in publicly challenging the administration over it.

But Mr. Comey’s testimony has given Democrats an opening to argue that they are focusing on the legal issues involved in the program, rather than on the merits of monitoring the phone calls of suspected terrorists.

“The Comey testimony moved this front and center,” said Senator Charles E. Schumer, the New York Democrat who is a member of the Judiciary Committee. “Alarm bells went off. His testimony made it clear that there had been an effort to circumvent the law.”