The Supreme Court on Monday struck down a provision of a campaign financing system in Arizona that gives extra cash to publicly funded candidates who face privately funded rivals and independent groups.

The 5-4 ruling is the latest in a series of decisions by the court's conservative majority upending campaign finance laws.

The Arizona law was passed in the wake of a public corruption scandal and was intended to reward candidates who forgo raising campaign cash, even in the face of opponents' heavy spending fueled by private money.

The court said the law violates the First Amendment.

"Laws like Arizona's matching funds provision that inhibit robust and wide-open political debate without sufficient justification cannot stand," Chief Justice John Roberts said in the court's majority opinion.

At least four other states, Maine, New Mexico, North Carolina and Wisconsin, have similar "trigger" provisions that affect some political races, and could be vulnerable.

Justice Elena Kagan read her dissent aloud in court Monday, saying the law was a reasonable response to political scandal. She said that by providing candidates with additional money, the law actually provided for more, not less, political speech.

Arizonans "passed a law designed to sever political candidates' dependence on large contributors," Kagan said. "It put into effect a public financing system that attracted large numbers of candidates at a sustainable cost to the state's taxpayers."

This case follows other recent rulings striking down campaign finance laws. Among those were last year's Citizens United decision that removed most limits on election spending by corporations and organized labor, and a 2008 decision that voided the federal "millionaire's amendment" to increase contribution limits for congressional candidates facing wealthy opponents.