WASHINGTON – A major ruling Thursday by the U.S. Supreme Court could change how presidential and congressional campaigns are funded, possibly opening the floodgates of money from corporations, labor unions and other groups.

The ruling is a blow to activists who have tried to limit the role of special interests in American politics, but critics of the limits have argued that they amount to an unconstitutional restraint of free speech and the 5-4 court majority agreed.

The court threw out a 63-year-old law designed to restrain the influence of big business and unions on elections Thursday, ruling that corporations may spend as freely as they like to support or oppose candidates for president and Congress. The decision could drastically alter who gives and gets hundreds of millions of dollars in this November’s crucial congressional elections.

The court overturned two of its own decisions as well as the decades-old law that said companies and unions can be prohibited from using money from their general treasuries to produce and run their own campaign ads. The decision threatens similar limits imposed by 24 states.

It leaves in place a prohibition on direct contributions to candidates from corporations and unions.

“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.

Strongly disagreeing, Justice John Paul Stevens said in his dissent, “The court’s ruling threatens to undermine the integrity of elected institutions around the nation.”

Other justices in the court’s liberal wing, 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 this year’s congressional elections.

The opinion goes to the heart of laws dating back to the Gilded Age when Congress passed the Tillman Act in 1907 banning corporations from donating money directly to federal candidates. Though that prohibition still stands, the same can’t be said for much of the century-long effort that followed to separate politics from corporate money.

The decision’s most immediate effect is to permit corporate and union-sponsored political ads to run right up to the moment of an election, and to allow them to call for the election or defeat of a candidate. In presidential elections and in highly contested congressional contests, that could mean a dramatic increase in television advertising competing for time and public attention.

In the long term, corporations, their industry associations and labor unions are free to tap their treasuries to assist candidates, although the spending may not be coordinated with the candidates.

“It’s going to be the Wild Wild West,” said Ben Ginsberg, a Republican attorney who has represented several party presidential campaigns. “If corporations and unions can give unlimited amounts … it means that the public debate is significantly changed with a lot more voices and it means that the loudest voices are going to be corporations and unions.”

Sen. Mitch McConnell, the Senate Republican leader who filed the first lawsuit challenging the McCain-Feingold law, praised the court for “restoring the First Amendment rights” of corporations and unions. “By previously denying this right, the government was picking winners and losers,” McConnell said. The First Amendment of the U.S. Constitution guarantees freedom of speech, among other rights.

Chief Justice John Roberts and Justices Samuel Alito, Antonin Scalia and Clarence Thomas joined Kennedy to form the majority in the main part of the case.

Roberts, in a separate opinion, said that upholding the limits would have restrained “the vibrant public discourse that is at the foundation of our democracy.”

Kennedy, who dissented from the rulings the court overturned Thursday, said, “No sufficient government interest justifies limits on the political speech of nonprofit or for-profit corporations.”

Stevens, in a 90-page opinion that dwarfed Kennedy’s, complained that the court majority overreached by throwing out earlier Supreme Court decisions that had not been at issue when this case first came to the court.

“Essentially, five justices were unhappy with the limited nature of the case before us, so they changed the case to give themselves an opportunity to change the law,” Stevens said.

The case began when a conservative group, Citizens United, made a 90-minute movie that was very critical of Hillary Rodham Clinton as she sought the Democratic presidential nomination. Citizens United wanted to air ads for the anti-Clinton movie and distribute it through video-on-demand services on local cable systems during the 2008 Democratic primary campaign.

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