The other dissenters were Justices David H. Souter, Stephen G. Breyer and Ruth Bader Ginsburg.

Chief Justice John G. Roberts Jr. joined the majority opinion, as did Justices Antonin Scalia, Clarence Thomas and Samuel A. Alito Jr. Justice Alito was not on the court when the case, Garcetti v. Ceballos, No. 04-473, was argued last October. The case had not been decided by the time Justice Sandra Day O'Connor retired and Justice Alito took his seat in January.

When the court ordered a second argument, held on March 21, the implication was that the eight remaining justices who had heard the case the first time were deadlocked and that Justice Alito would break the tie. The reality may have been more complex.

Justice Breyer made it clear in his separate dissenting opinion that he was satisfied with neither Justice Kennedy's majority opinion nor Justice Souter's principal opinion for the other three dissenters, and his vote may have been uncertain until late in the process. Justice Breyer would have emphasized the fact that as a lawyer, the plaintiff, Richard Ceballos, had special ethical obligations to speak up if he found problems with a pending prosecution and so deserved special constitutional protections.

In his lawsuit, Mr. Ceballos claimed that he was given an unwelcome transfer and was denied a promotion after taking several steps to complain about the apparent unreliability of an affidavit that had provided the basis for a search warrant in a case the office was prosecuting. He told his supervisors of his concerns, and wrote a memorandum recommending dismissal of the case. There was a heated meeting, after which his concerns were rejected.

Mr. Ceballos later testified for the defense at a court hearing, and spoke about the issue at a meeting of the Mexican American Bar Association.

The Supreme Court's focus, however, was entirely on the initial internal memorandum because the United States Court of Appeals for the Ninth Circuit, in ruling for Mr. Ceballos, found that the document was constitutionally protected speech and did not then go on to consider the constitutional status of his other actions.

In overturning the Ninth Circuit's judgment, Justice Kennedy's opinion said that in writing the memorandum, Mr. Ceballos was simply carrying out the normal duties of his job, which included advising his supervisors on how to proceed with pending cases.