WASHINGTON, Nov. 6  When Mark Klein, then an AT&T technician in San Francisco, stumbled on a secret room apparently reserved for the National Security Agency inside an AT&T switching center, he hardly expected to be caught up in a national debate over the proper balance between American civil liberties and national security.

But four years later, Mr. Klein’s discovery has led to a spate of class-action lawsuits against the nation’s largest telephone companies. The threat posed to the telecommunications industry by those suits has prompted the Bush administration to push Congress to grant companies legal immunity for their secret cooperation in the N.S.A.’s program of eavesdropping without warrants. With many Democrats in Congress seemingly willing to grant the legal protection, Mr. Klein has come to Washington to fight back.

Mr. Klein, 62 and now retired, will begin meeting Wednesday with staff members on the Senate and House Judiciary Committees to try to persuade them to put a brake on the immunity legislation. He says the phone companies do not deserve the legal protection.

“I think they committed a massive violation not only of the law but of the Constitution,” he said. “That’s not the way the Fourth Amendment is supposed to work.”