By Brent Kendall, Of DOW JONES NEWSWIRES

WASHINGTON -(Dow Jones)- A divided U.S. Supreme Court on Monday struck down an Arizona campaign-finance law that offered extra public funding to state political candidates who faced well-heeled opponents.

Arizona said its public financing system promoted free speech by giving candidates the opportunity to run for office without depending on private political donors. But the law's challengers--five conservative politicians and two political action committees--said the law stifled free speech. They argued that, when they raised and spent money to promote their messages, their speech was punished because it triggered government subsidies to their rivals.

The Supreme Court's conservative majority agreed with the challengers in a 5-4 ruling.

"Laws like Arizona's matching funds provision that inhibit robust and wide- open political debate without sufficient justification cannot stand," Chief Justice John Roberts wrote for the majority in a 30-page ruling.

The court's four liberal members dissented.

"Like citizens across this country, Arizonans deserve a government that represents and serves them all," Justice Elena Kagan wrote in dissent. "And no less, Arizonans deserve the chance to reform their electoral system so as to attain that most American of goals."