The decision Monday, the Roberts court’s first direct look at public campaign financing, concerned only systems that use matching funds, as opposed to lump-sum grants. About a third of the states have some form of public financing, as does the federal government for presidential elections.

“We do not today call into question the wisdom of public financing as a means of funding political candidacy,” Chief Justice Roberts wrote. “That is not our business.”

Supporters of the law said the decision could have been worse. “Chief Justice Roberts at least recognized that public financing is a valid constitutional option,” said Monica Youn, a lawyer with the Brennan Center for Justice, which represented one of the defendants in the case.

As a consequence of the decision, states and municipalities are now blocked from using a method of public financing that is simultaneously likely to attract candidates fearful that they will be vastly outspent and sensitive to avoiding needless government expense.

“The government can still use taxpayer funds to subsidize political campaigns, but it can only do that in a manner that provides an alternative to private financing” said William R. Maurer, a lawyer with the Institute for Justice, which represented several challengers of the law. “It cannot create disincentives.”

Chief Justice Roberts said that all escalating matching funds placed an unconstitutional burden on politicians who chose not to participate. But he added that Arizona’s system also created problematic asymmetries and anomalies. Candidates with several opponents could generate multiple subsidies every time they spent money, and spending from unaffiliated supporters could do the same.

Justice Antonin Scalia, Anthony M. Kennedy, Clarence Thomas and Samuel A. Alito Jr. joined the majority opinion.

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Three years ago, in Davis v. Federal Election Commission, another 5-to-4 decision with the same justices in the majority, the court struck down a superficially similar federal law known as the “millionaire’s amendment.” That law allowed candidates to raise amounts over the usual contribution limits when rich opponents spent more than a given amount of their own money.

Justice Alito, writing for the majority, said the law imposed “an unprecedented penalty on any candidate who robustly exercises” free speech rights guaranteed by the First Amendment.

Chief Justice Roberts said the logic of the Davis decision required the court to strike down the Arizona law. Indeed, he said, it is one thing for the government to allow candidates to seek additional contributions and another for the government to send a check.

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“The cash subsidy, conferred in response to political speech, penalizes speech to a greater extent and more directly than the millionaire’s amendment in Davis,” Chief Justice Roberts wrote.

The decision concerned two consolidated cases, Arizona Free Enterprise Club v. Bennett, No. 10-238, and McComish v. Bennett, No. 10-239. It was the fifth ruling from the Roberts court cutting back on the government’s ability to regulate campaign finance.

In a dissent summarized from the bench, Justice Elena Kagan said the Arizona law advanced First Amendment values.

“What the law does — all the law does — is fund more speech,” she wrote. Justices Ruth Bader Ginsburg, Stephen G. Breyer and Sonia Sotomayor joined the dissent.

“Arizona, remember, offers to support any person running for state office,” Justice Kagan wrote. The candidates who challenged the law declined to accept that help, she said.

“So they are making a novel argument: that Arizona violated their First Amendment rights by disbursing funds to other speakers even though they could have received (but chose to spurn) the same financial assistance,” Justice Kagan wrote. “Some people might call that chutzpah.”

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The Davis decision, Justice Kagan wrote, involved a different issue, as it concerned a law that raised contribution limits disproportionately.

The majority and dissent disagreed about whether the Arizona law was supported by a permissible government rationale.

Chief Justice Roberts wrote that its main purpose was to level the playing field for political speech, which several earlier decisions have said is an improper goal.

“It is not legitimate for the government to attempt to equalize electoral opportunities in this manner,” he wrote. “And such basic intrusion by the government into the debate over who should govern goes to the heart of First Amendment values.”

“ ‘Leveling the playing field,’ ” Chief Justice Roberts wrote, “can sound like a good thing. But in a democracy, campaigning for office is not a game.”

Justice Kagan countered that the main purpose of the law was to root out corruption and the appearance of corruption by encouraging candidates to participate in public financing systems, a goal the Supreme Court has endorsed.

“Like citizens across this country, Arizonans deserve a government that represents and serves them all,” she wrote. “And no less, Arizonans deserve the chance to reform their electoral system so as to attain that most American of goals. Truly, democracy is not a game.”