Debra Burlingame, an organizer of the December rally, whose brother, Charles F. Burlingame III, was the pilot of the hijacked plane that was crashed into the Pentagon on Sept. 11, said: “He’s done a lot of heavy lifting on our behalf. This fight gets very tiring, and Andy is one of those people that truly inspires and keeps me going.”

Fellow alumni of the United States attorney’s office for the Southern District of New York have mixed views about Mr. McCarthy, who also writes on topics like abortion and overhauling health care. “His critics view him as a right-wing blogger,” said Anthony S. Barkow, a former terrorism prosecutor in the office who now runs a center on criminal law at New York University.

Mr. Barkow said he had stopped reading Mr. McCarthy on topics other than national security. “I have to give him credit for being willing to reject his past a bit,” he said, “and be out there so vehemently against something he was so integrally a part of.”

Through a spokesman, Attorney General Eric H. Holder Jr. declined to comment about Mr. McCarthy. When asked about him during an appearance before the Senate Judiciary Committee in November, Mr. Holder replied that he was there “to talk about facts and evidence, real American values, and not the kinds of polemics that he seems prone to.”

“I’m not worried about Mr. McCarthy,” Mr. Holder said.

Mr. McCarthy now writes regularly for National Review. He makes extensive public appearances, and he wrote a 2008 book on terrorism policy called “Willful Blindness.” He said in interviews that the evolution of his views came during his stint as a prosecutor.

A graduate of Cardinal Hayes High School in the Bronx, Mr. McCarthy, the oldest of six children, was 13 when his father died. After graduating from Columbia College, he attended New York Law School at night while working days as a deputy marshal in the witness protection program. He later became a paralegal in the United States attorney’s office.

Hired as a prosecutor in 1986, he worked on the Pizza Connection heroin trial. He was running the office’s general crimes unit in 1993 when the World Trade Center was bombed and the conspiracy involving the sheik, Omar Abdel Rahman, was uncovered.