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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. But, giving a glimmer of hope to advocates of limiting the role of money in politics, the court did not launch a broad attack on taxpayer-funded campaigns.

Instead, Chief Justice John Roberts' majority opinion dwelled on the so-called trigger mechanism in Arizona law that provides differing levels of money to publicly funded candidates based on the spending by privately funded rivals and independent groups.

The 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 trigger violates the First Amendment, but left in place the rest of Arizona's public financing system.

"Laws like Arizona's matching funds provision that inhibit robust and wide-open political debate without sufficient justification cannot stand," Roberts said.

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."

Publicly funded candidates receive money from the $3 that Americans check off on their tax returns in exchange for abiding by various spending limits.

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.

Roberts said the outcome in the Arizona case largely followed from the decision voiding the "millionaire's amendment."

But the chief justice — joined by Justices Samuel Alito, Anthony Kennedy, Antonin Scalia and Clarence Thomas — said nothing in the court's decision should be read as an attack on public financing generally.

Justices Stephen Breyer, Ruth Bader Ginsburg and Sonia Sotomayor signed onto Kagan's dissent.

Groups in favor of laws like Arizona's criticized the outcome, but took some solace from Roberts' rhetoric on public financing.

"After the court's ruling today, one key fact is clear. Public financing remains constitutionally strong," said Michael Waldman, executive director of New York University's Brennan Center. The center represented Arizona's Clean Elections Institute at the high court.

William Maurer, an attorney with the Institute for Justice representing the challengers, said the court reaffirmed its opposition to campaign spending laws that seek to level the playing field.

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