“She and I, in terms of policy issues, growth, energy, social issues, we certainly agree on the overwhelming number of issues,” said Senator Ron Wyden of Oregon, who along with Senator Mark Udall of Colorado has been a voice of dissent on the Intelligence Committee. “But, you know, families disagree.”

He added, “Here’s why we’re going to win: Because our side believes that security and liberty are not mutually exclusive.”

Ms. Feinstein, never one to shy away from uncomfortable disagreements with colleagues, observed that Mr. Wyden and Mr. Udall were in the minority in their beliefs. “Two people out of 15,” she said matter of factly.

“What do you think would happen if Najibullah Zazi was successful?” she asked, referring to the man who pleaded guilty to plotting to bomb the New York City subway. Intelligence officials have said N.S.A. e-mail surveillance helped them catch Mr. Zazi. “There would be unbridled criticism,” she said. “Didn’t we learn anything? Can’t we protect our homeland? What good is intelligence if we can’t stop this? So there’s a flip side to all this.”

For those who have worked alongside Ms. Feinstein, her resolve comes as no surprise. She angered Senator Harry Reid of Nevada, the majority leader, this year when she publicly criticized his decision to drop her assault weapons ban from a package of gun control bills. “Show some guts,” she implored her colleagues from the Senate floor as they were about to vote her proposal down.

She has always had a defiant streak. About a decade ago, Ms. Feinstein decided to add a playful feminist twist to one of the favorite institutions of a colleague then, Trent Lott of Mississippi, who had revived it when he was majority leader: Seersucker Thursday, when male senators wore seersucker suits to honor a Southern sartorial tradition. Ms. Feinstein bought all the female senators seersucker suits of their own.

“She is gutsy,” said Senator Susan Collins, Republican of Maine and a colleague on the Intelligence Committee. “She knows more — and I don’t mean that in a facetious way. She really knows the details,” Ms. Collins said. “She’s studied the program. She helped to strengthen the safeguards. She gets briefed regularly.”