The decision upheld an appeals court ruling that a Wisconsin anti-abortion group should have been allowed to air ads during the final two months before the 2004 elections. The law unreasonably limits speech and violates the group's First Amendment rights, the court said.

The law, a provision in the 2002 campaign finance act, banned corporations and unions from paying for political "issue ads" that mentioned a candidate for federal office within 60 days of a federal election and 30 days of a primary or caucus.

"Discussion of issues cannot be suppressed simply because the issues may also be pertinent in an election," Chief Justice John Roberts wrote for the majority. "Where the First Amendment is implicated, the tie goes to the speaker, not the censor."

The law's provision not only applies to organized labor and business corporations, but also to any special interest that operated as a corporation, such as the U.S. Chamber of Commerce, the National Rifle Association and the Sierra Club -- groups frequently involved in elections. The ruling does not change a ban on ads that specifically call for the election or defeat of a candidate.

"This decision helps put the NRA in the same playing field as the politicians and the big media conglomerates going into the 2008 elections," Wayne LaPierre, executive vice president of the National Rifle Association, said in an interview.

A first test of the impact of the court's opinion could come as early as December, a month before presidential caucuses and primaries in Iowa, Nevada, New Hampshire and South Carolina open the nomination process.

The case addressed television ads by Wisconsin Right to Life that asked voters to contact the state's two senators, Democrats Russ Feingold and Herb Kohl, and urged them not to filibuster President Bush's judicial nominees. Because Feingold was up for re-election at the time, the Federal Election Commission said the ads violated the 2002 campaign finance law that Feingold and Sen. John McCain helped write.

Campaign finance experts said that while the court's decision, written by Roberts, applied specifically to the Wisconsin case, the ruling has far-reaching implications.

In making his argument, Roberts said ads that focus on a legislative issue, take a position on an issue and urge the public to contact a public official is a legitimate "issue ad" that should run no matter how close to the election. Whether the ads intend to affect an election, Roberts said, does not matter.

"I don't think (Chief) Justice Roberts is naive," Richard L. Hasen, a professor at Loyola Law School Los Angeles, said. "He knows full well that the test that the court has articulated today will lead to a great deal of corporation- and union- funded election advertising."

Writing more broadly, Roberts said the court has upheld contribution limits and some limits on expenditures in the interest of preventing corruption and the appearance of corruption. But he said that interest "must be stretched yet another step" in the Wisconsin case.

"Enough is enough," Roberts wrote.

Some campaign finance experts said Roberts' phrase carried a deeper meaning.

"For anybody who is looking for trouble ahead, that's certainly one of the places you would look," Robert Bauer, a campaign finance lawyer who is representing Barack Obama's presidential campaign. "He wants to remind everybody that having reviewed the entire line of argument up to this point he had quite enough of it."

The court's decision does not address the more far-reaching component of the campaign finance law -- it's ban on the ability of political parties to raise unlimited and unrestricted amounts of money from unions, corporations and wealthy donors.

But some campaign finance experts said that by opening the door to corporate and union-financed advertising, the court set the stage for a broader challenge to the law.

"Fundamentally what this case does is destabilize the state of campaign finance law as it existed when Justice (Sandra Day) O'Connor was on the court," said Nathaniel Persily, professor of law and political science at Columbia Law School.

The decision means the FEC likely will have to step in and write specific rules about advertising that reflects the court's opinion. The commission may face pressure to act before the end of the year.

The decision is a setback for McCain, who is now running for president. McCain has come under criticism from conservatives for attempting to restrict political money and political advertising.

"Obviously, I regret that decision, but it was very narrow," McCain told reporters in Columbia, S.C.

Presidential rival Mitt Romney cheered the ruling: "It's the beginning of an opening, I believe, to remove McCain-Feingold and its provisions that affect free speech and hopefully its broader provisions." Another candidate, Rudy Giuliani, praised the ruling as "a welcome victory for free speech and personal liberty." He previously had expressed support for the campaign finance reform act.

The court's majority was itself divided on the issue, with Roberts and Justice Samuel Alito saying only that the Wisconsin group's ads are not the equivalent of explicit campaign ads. They specifically said they were not overruling a 2003 court opinion that upheld the campaign finance law provision.

The three other justices that formed the majority -- Anthony Kennedy, Antonin Scalia and Clarence Thomas -- would have overruled the court's 2003 decision.

Justice David Souter, joined by his three liberal colleagues, said in his dissent that the court "effectively and, unjustifiably, overruled" the earlier decision.

Justices Stephen Breyer, Ruth Bader Ginsburg and John Paul Stevens joined Souter's dissent.