WASHINGTON – The Supreme Court scaled back protections for government workers who blow the whistle on official misconduct yesterday, a 5-4 decision in which new Justice Alito cast the deciding vote.

In a victory for the Bush administration, justices said the 20 million public employees do not have free-speech protections for what they say as part of their jobs.

Critics predicted the impact would be sweeping, from silencing police officers who fear retribution for reporting department corruption, to subduing federal employees who want to reveal problems with government hurricane preparedness or terrorist-related security.

Supporters said that it will protect governments from lawsuits filed by disgruntled workers pretending to be legitimate whistleblowers.

The ruling was perhaps the clearest sign yet of the Supreme Court’s shift with the departure of moderate Justice O’Connor and the arrival of Justice Alito.

A year ago, Justice O’Connor authored a 5-4 decision that encouraged whistleblowers to report sex discrimination in schools. The current case was argued in October but not resolved before her retirement in late January.

A new argument session was held in March with Justice Alito on the bench. He joined the court’s other conservatives in yesterday’s decision, which split along traditional conservative-liberal lines.

Exposing government misconduct is important, Justice Kennedy wrote for the majority. “We reject, however, the notion that the First Amendment shields from discipline the expressions employees make pursuant to their professional duties,” Justice Kennedy said.

The ruling overturned an appeals court decision that said Los Angeles County prosecutor Richard Ceballos was constitutionally protected when he wrote a memo questioning whether a county sheriff’s deputy had lied in a search warrant affidavit. Mr. Ceballos had filed a lawsuit claiming he was demoted and denied a promotion for trying to expose the lie.

Justice Kennedy said if the superiors thought the memo was inflammatory, they had the authority to punish him.

“Official communications have official consequences, creating a need for substantive consistency and clarity. Supervisors must ensure that their employees’ official communications are accurate, demonstrate sound judgment, and promote the employer’s mission,” Justice Kennedy wrote.

The chairman of the National Whistleblower Center, Stephen Kohn, said: “The ruling is a victory for every crooked politician in the United States.”

Justice Souter’s lengthy dissent sounded like it might have been the majority opinion if Justice O’Connor were still on the court. “Private and public interests in addressing official wrongdoing and threats to health and safety can outweigh the government’s stake in the efficient implementation of policy,” he wrote.

Justice Souter was joined by Justices Stevens and Ginsburg. Justice Breyer also supported Mr. Ceballos, but on different grounds.

The ruling upheld the position of the Bush administration, which had joined the district attorney’s office in opposing absolute free-speech rights for whistleblowers. President Bush’s two nominees, Justice Alito and Chief Justice Roberts, signed onto Kennedy’s opinion but did not write separately.

“It’s a very frightening signal of dark times ahead,” the legal director for the Government Accountability Project, Tom Devine, said.

An employment attorney, Dan Westin, said that Justice Kennedy’s ruling frees government managers to make necessary personnel actions, like negative performance reviews or demotions, without fear of frivolous lawsuits.

“I don’t think he’s unleashed a wave of terminations,” Mr. Westin said.

A Washington lawyer, Gene Schaerr, a former government attorney, said the ruling keeps courts from meddling in the business of local governments. “It’s not the role of the First Amendment to be an all-purpose whistleblower law,” he said.

The court’s decision immediately prompted calls for Congress to strengthen protections for workers.

Justice Kennedy said that government workers “retain the prospect of constitutional protection for their contributions to the civic discourse.” They do not, Justice Kennedy said, have “a right to perform their jobs however they see fit.”