SAN DIEGO — Representative Duncan D. Hunter, Republican of California, pleaded guilty in federal court on Tuesday to conspiracy to steal campaign funds. The guilty plea threatened to end what had once been seen as a promising career in conservative politics.

Mr. Hunter, who had insisted for more than a year that the charges against him were unfounded, stood in the packed courtroom in San Diego beside his lawyer, Paul Pfingst, and listened expressionlessly as the charge was read aloud, accusing him of using campaign funds to pay personal expenses. Other than answering “guilty” to the judge’s question about his plea, Mr. Hunter did not speak during the hearing, which lasted less than 10 minutes.

As he left the courthouse afterward, he told a crowd of reporters that he had “made mistakes,” but he did not respond to questions.

Emily Allen, one of three federal prosecutors on the case, rejected that characterization, saying after the hearing that “this is not a case about mismanagement or accounting errors or mistakes” and commending the F.B.I. for “rooting out corruption.”