By a 5-4 vote, the court on Thursday overturned a 20-year-old ruling that said corporations can be prohibited from using money from their general treasuries to pay for their own campaign ads. The decision, which almost certainly will also allow labor unions to participate more freely in campaigns, threatens similar limits imposed by 24 states.
It leaves in place a prohibition on direct contributions to candidates from corporations and unions.
Critics of the stricter limits have argued that they amount to an unconstitutional restraint of free speech, and the court majority apparently agreed.
"The censorship we now confront is vast in its reach," Justice Anthony Kennedy said in his majority opinion, joined by his four more conservative colleagues.
However, Justice John Paul Stevens, dissenting from the main holding, said, "The court's ruling threatens to undermine the integrity of elected institutions around the nation."
Justices Ruth Bader Ginsburg, Stephen Breyer and Sonia Sotomayor joined Stevens' dissent, parts of which he read aloud in the courtroom.
The justices also struck down part of the landmark McCain-Feingold campaign finance bill that barred union- and corporate-paid issue ads in the closing days of election campaigns.
Advocates of strong campaign finance regulations have predicted that a court ruling against the limits would lead to a flood of corporate and union money in federal campaigns as early as November's congressional elections.
The decision, written by Justice Anthony Kennedy, removes limits on independent expenditures that are not coordinated with candidates' campaigns.
The case also does not affect political action committees or PACS, which mushroomed after 1970s post-Watergate laws set the first limits on contributions by individuals to candidates. Corporations, unions and others may create PACs to contribute directly to candidates, but they must be funded with voluntary contributions from employees, members and other individuals, not by corporate or union treasuries.