WASHINGTON — In its first campaign-finance decision since its 5-to-4 ruling in the Citizens United case last year, the Supreme Court on Monday struck down an Arizona law that provided escalating matching funds to candidates who accept public financing.

The vote was again 5 to 4, with the same five justices in the majority as in the Citizens United decision. The majority said the law violated the First Amendment rights of candidates who raise private money. Such candidates, the majority said, may be reluctant to spend money to speak if they know that it will give rise to counterspeech paid for by the government.

“Laws like Arizona’s matching funds provision that inhibit robust and wide-open political debate without sufficient justification cannot stand,” Chief Justice John G. Roberts Jr. wrote for the majority.

The Arizona Citizens Clean Elections Act, passed by a ballot initiative in 1998, gave public money to candidates who agreed to limit their personal spending to $500, participate in at least one debate and return unspent money. Such candidates received initial grants and then more money based on the amounts spent by privately financed opponents and by independent groups supporting them.