The court, split 5-4, 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.
The decision could lead to a bigger role for corporations, unions and other interest groups in the 2008 presidential and congressional elections.
"This is a defeat for the McCain-Feingold legislation that was the subject of so much political and legal discussion a few years ago," said CBS News legal analyst Andrew Cohen.
"And it is in many ways part of a return to the old ways, when there were fewer restrictions on the way people spend money when it comes to campaigns."
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.
Chief Justice John Roberts, joined by his conservative allies, wrote a majority opinion upholding the appeals court ruling.
The majority itself was divided in how far justices were willing to go in allowing issue ads.
Three justices, Anthony Kennedy, Antonin Scalia and Clarence Thomas, would have overruled the court's 2003 decision upholding the constitutionality of the provision.
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 court, differently composed, upheld large portions of the law in its 2003 decision, including the provision in question in the current case.
On Monday, 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.
Justices Stephen Breyer, Ruth Bader Ginsburg and John Paul Stevens joined Souter's dissent.
The Bush administration urged the court to ban the ads, arguing that they were meant to influence the elections, not lobby the senators.
But Roberts said, "Discussion of issues cannot be suppressed simply because the issues also may be pertinent in an election. Where the First Amendment is implicated, the tie goes to the speaker, not the censor."
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.
The consolidated case is Federal Election Commission v. Wisconsin Right to Life, 06-969, and McCain v. Wisconsin Right to Life, 06-970.