The U.S. Supreme Court issued a ruling today in a case that will impact Maine’s public campaign financing system.

The court ruled 5-4 against an Arizona law – which is similar to Maine’s – that provides additional money to publicly financed lawmakers when they are outspent by privately funded opponents or outside political groups.

The court said the law violates the constitutional right to free speech and that Arizona was illegally trying to “level the playing field.” Supporters of the law had said there was a compelling state interest in providing equal footing to taxpayer-funded candidates.

Maine’s Clean Elections Act law has a similar provision that provides matching funds that will have to be altered to preserve the system.

About 80 percent of Maine legislators run as publicly financed candidates, according to the Maine ethics commission. Matching funds account for about 24 percent of all Clean Elections money spent in the legislative races, according to Jonathan Wayne, executive director of the commission.

The 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.

Besides Maine, at least three other states – 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 today, 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.