The court's 5-4 ruling could become a significant factor in the upcoming presidential primaries, giving interest groups a louder and more influential voice in the closing days before those contests as well as the general election.
The decision upheld an appeals court ruling that an 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.
"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 case involved advertisements that Wisconsin Right to Life was prevented from broadcasting. The ads asked voters to contact the state's two senators, Democrats Russ Feingold and Herb Kohl, and urge them not to filibuster President Bush's judicial nominees.
Feingold, a co-author of the campaign finance law, was up for re-election in 2004.
The provision in question was aimed at preventing the airing of issue ads that cast candidates in positive or negative lights while stopping short of explicitly calling for their election or defeat. Sponsors of such ads have contended they are exempt from certain limits on contributions in federal elections.
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.
"The ruling could have important implications for the 2008 presidential election and could reorder the advertising strategies of corporate America and labor unions over the next two years," said Michael Toner, a former chairman of the Federal Election Commission, which oversees campaign finance law.
The decision is a setback for Sen. John McCain, R-Ariz., who helped write the 2002 campaign finance legislation with Feingold that contained the advertising provision. McCain, now a presidential candidate, has come under criticism from conservatives for attempting to restrict political money and political advertising.
"It is regrettable that a split Supreme Court has carved out a narrow exception by which some corporate and labor expenditures can be used to target a federal candidate in the days and weeks before an election," McCain said in a statement.
The court's decision, however, has no effect on 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.
"Fortunately," McCain said, "that central reform still stands as the law."
Republican presidential candidate Mitt Romney, who has been critical of McCain's stance, promptly hailed the court's decision.
"McCain-Feingold was a poorly-crafted bill," Romney said in a statement. "Today's decision restores, in part, to the American people a right critical to their freedom of political participation and expression."
Roberts and Justice Samuel Alito said only that the Wisconsin group's ads are not the equivalent of explicit campaign ads and are not covered by the court's 2003 decision.
That means the FEC will likely have to step in and write specific rules about such advertising that reflect the court's opinion.
Justice David Souter, joined by his three liberal colleagues, said in his dissent that the court "effectively and, unjustifiably, overruled" the earlier decision.
The ads could have been run, Souter pointed out, had they been paid for out of the group's political action committee, which is subject to federal campaign finance limits. Or Feingold's name could have been omitted, he said.
"Thus, what is called a 'ban' on speech is a limit on the financing of electioneering broadcasts by entities ... that insist on acting as conduits from the campaign war chests of business corporations," Souter said.
The Bush administration urged the court to ban the ads, arguing that they were meant to influence the elections, not lobby the senators.
An array of interest groups across the political spectrum sought the outcome the court reached Monday. They include: the American Civil Liberties Union, the National Rifle Association, labor unions and business groups.