WASHINGTON — The Supreme Court yesterday unanimously upheld a Nevada ethics law that governs when lawmakers should refrain from voting on official business because they might have a conflict of interest.

The court reversed a Nevada Supreme Court decision that said elected officials have a constitutional right to vote on official business and that the state law violated that right.

The decision came in the case of Michael Carrigan, a Sparks, Nev., council member who voted on a casino project even though his campaign manager served as a project consultant.

Justice Antonin Scalia, writing for the court, said an elected official’s vote “is not his own speech but a mechanical function of government — the commitment of his apportioned share of the legislature’s power to the passage or defeat of a particular proposal.’’

Scalia said conflict-of-interest rules similar to Nevada’s “have been commonplace for over 200 years.’’

The Nevada Commission on Ethics found that Carrigan’s vote violated a provision of the ethics law that lays out various relationships that should disqualify an official from voting, including the official’s relatives and business associates.

The ethics panel said Carrigan should have abstained from voting because his friend and campaign manager, Carlos Vasquez, also worked as a consultant for the Red Hawk Land Co., which was backing a casino project.

Carrigan challenged that ruling, leading to the state court decision striking down part of the law.

Nevada and 14 other states argued that ethics laws around the nation would be threatened if the high court were to endorse the Nevada court ruling.

News organizations and press freedom groups filed a brief supporting the Nevada ethics commission.

Scalia said Carrigan may be able to make additional arguments against the law in state court.

Justice Samuel Alito wrote separately to say he did not accept all of Scalia’s reasoning, including the view that a lawmaker’s vote isn’t protected speech. Alito referred to an 1807 vote cast by John Quincy Adams that may have cost the future president his US Senate seat.

“Voting has an expressive component in and of itself,’’ Alito wrote. “The court’s strange understanding of the concept of speech is shown by its suggestion that the symbolic act of burning the American flag is speech but John Quincy Adams calling out ‘yea’ on the Embargo Act was not.’’

In other action yesterday:

■ The high court ordered a lower court to reconsider its decision to release a criminal on parole. It threw out a lower court decision ordering John Pirtle and other California state inmates released from prison on parole.

Pirtle was convicted of killing his wife, and the parole board started denying him parole in 2002. Pirtle sued in federal court, saying his parole was denied without any proof that he posed a danger if he got out.

The lower courts agreed with him and ordered him and other prisoners in similar situations released on parole. The high court threw out that decision in a summary judgment and ordered the US Court of Appeals for the Ninth Circuit in San Francisco to reconsider it.

■ The justices dismissed a challenge by a Mexican-born man to a citizenship law that treats men and women differently.

The court split, 4-4, in the case of Ruben Flores-Villar of San Diego, an outcome that upholds his criminal conviction on immigration charges. The tie vote resolves Flores-Villar’s case, but sets no national precedent. Justice Elena Kagan did not take part because she worked on the case as solicitor general.

© Copyright 2011 Globe Newspaper Company.