In a 5-4 decision, the U.S. Supreme Court invalidated a key part of Arizona's so-called Clean Elections program that triggered state payments of matching funds for publicly financed candidates whenever their privately funded opponents outspent them.

The high court ruled Monday the nearly dollar-for-dollar matching-funds mechanism violated the free speech protections of the First Amendment by deterring or diminishing the effectiveness of the speech of candidates who opt out of the public finance system.

"The whole point of the First Amendment is to protect speakers against unjustified government restrictions on speech," Chief Justice John Roberts wrote in the majority opinion. "When it comes to speech, the speaker is sovereign."

The decision marks the third time in four years the high court has invalidated campaign finance reform laws.

"Leveling the playing field can sound like a good thing," Chief Justice Roberts wrote in this week's decision. "But in a democracy, campaigning for office is not a game. It is a critically important form of speech."

He added: "The First Amendment embodies our choice as a nation that, when it comes to such speech, the guiding principle is freedom - the ‘unfettered interchange of ideas'-not whatever the State may view as fair."

In a dissent, Justice Elena Kagan said the majority was wrong in finding that the Arizona provision posed a burden on political speech. She said the matching-funds mechanism provides a viewpoint-neutral subsidy to political candidates and that it fosters more speech, not less.

"We have never, not once, understood

a viewpoint-neutral subsidy given to one speaker to constitute a First Amendment burden on another," Justice Kagan said. "Yet in this case, the majority says that the prospect of more speech - responsive speech, competitive speech, the kind of speech that drives public debate - counts as a constitutional injury."

The decision split the justices along conservative-liberal lines. Justice Kagan's dissent was joined by Justices Ruth Bader Ginsburg, Stephen Breyer, and Sonia Sotomayor.

The chief justice was joined in the majority by Justices Antonin Scalia, Anthony Kennedy, Clarence Thomas, and Samuel Alito.

At issue in the consolidated cases, Arizona Free Enterprise Club v. Bennett (10-238) and McComish v. Bennett (10-239), was a provision of Arizona law that provided a near dollar-for-dollar match to candidates participating in the public-finance system whenever a nonparticipating candidate outspent the initial public funding to his or her opponent.

The matching funds were meant to keep elections competitive between privately-funded and public-funded candidates.

Under the 1998 Citizens Clean Elections Act, candidates who are willing to forego private fundraising may finance their campaigns with state money. First they must qualify by demonstrating a certain level of support by raising a required number of $5 donations.

Once qualified, the publicly funded candidate receives a lump sum grant to pay for election expenses. This portion of the law was not under challenge.

The controversial part of the law deals with the mechanism enacted to ensure that privately funded candidates do not automatically outspend their competition. The matching funds were awarded to every candidate who agreed to participate in the public-funding option - not just the most competitive candidates. In addition, publicly funded candidates were to receive matching funds whenever an independent advocacy group spent money in support of a privately funded candidate or spent money in a way that opposed a publicly funded candidate.