By a 6-3 vote, the justices overturned the conviction of a former Indiana mayor who asked for and took a $13,000 payment from the owners of a local truck dealership after he helped them win $1.1 million in city contracts for the purchase of garbage trucks.

In ruling for the former mayor, the justices drew a distinction between bribery, which requires proof of an illegal deal, and a gratuity that can be a gift or a reward for a past favor. They said the officials may be charged and prosecuted for bribery, but not for simply taking money for past favors if there was no proof of an illicit deal.

“The question in this case is whether [the federal law] also makes it a crime for state and local officials to accept gratuities — for example, gift cards, lunches, plaques, books, framed photos or the like — that may be given as a token of appreciation after the official act. The answer is no,” said Justice Brett M. Kavanaugh, writing for the majority.

Kavanaugh said federal law “leaves it to state and local governments to regulate gratuities to state and local officials.”

Justices Elena Kagan, Sonia Sotomayor and Ketanji Brown Jackson dissented. “Officials who use their public positions for private gain threaten the integrity of our most important institutions,” Jackson said.

The ruling could have a broad impact. About 20 million local and state officials are covered by the federal anti-corruption law, including officials at hospitals and universities that receive federal funds.

Justice Department lawyers told the court that for nearly 40 years, the anti-bribery law has been understood to prohibit payments to officials that “rewarded” them for having steered contracts to the donors.

The Supreme Court justices have faced heavy criticism recently for accepting undisclosed gifts from wealthy patrons. Justice Clarence Thomas regularly took lavish vacations and private jet flights that were paid for by Texas billionaire Harlan Crow. Justice Samuel A. Alito Jr. took a fishing trip to Alaska in 2008 aboard a private plane owned by Paul Singer, a hedge fund billionaire.

The high court has long held that criminal laws restricting “illegal gratuities” to federal officials require proof that the gifts were given for a specific “official act,” not just because of the official’s position.

The Indiana mayor was charged and convicted of taking the $13,000 payment because of his role in helping his patrons win city contracts.

Congress in 1986 extended the federal bribery law to cover officials of state or local agencies that receive federal funds. The measure made it a crime to “corruptly solicit or demand … or accept … anything of value of $5,000 or more … intending to be influenced or rewarded in connection with any business or transaction.”

Prosecutors said James Snyder was heavily in debt and behind in paying his taxes when he became mayor of Portage, Ind., in 2012. The city needed new garbage trucks, and the mayor took over the required public bidding. He spoke regularly with two brothers who owned a local truck dealership that also had financial problems, and he designed the bidding process so that only their two new trucks would meet all of its standards. He also arranged to have the city buy an older truck that was on their lot.

Two weeks after the contracts were final, the mayor went to see the two brothers and told them of his financial troubles. They agreed to write him a check for $13,000 for undefined consulting services.

An FBI investigation led to Snyder’s indictment, his conviction and a 21-month prison sentence.

The former mayor argued that an after-the-fact gift should not be a crime, but he lost before a federal judge and the U.S. appeals court in Chicago.

The high court agreed to hear his appeal in Snyder vs. U.S. because appeals courts in Boston and New Orleans had limited the law to bribery only and not gratuities that were paid later.

In recent years, the Supreme Court has repeatedly limited the scope of public corruption laws and often in unanimous rulings. The common theme is that the justices concluded the prosecutions went beyond the law.

Last year, the court was unanimous in overturning the corruption convictions of two New York men who were former aides or donors to then-Gov. Andrew Cuomo, a Democrat. The court noted that one of the defendants convicted of taking illicit payments did not work for the state during that time.

Four years ago, the justices were unanimous in overturning the convictions of two aides to then-New Jersey Gov. Chris Christie, a Republican, who were charged with conspiring to shut down lanes to the George Washington Bridge into New York City. The court said they were wrongly convicted of fraud because they had not sought money or property, which is a key element of a fraud charge.

In 2016, the court overturned the corruption conviction of former Virginia Gov. Bob McDonnell, a Republican. While the governor took $175,000 in gifts from a business promoter, he took no official actions to benefit the donor, the court said.