WASHINGTON — The Supreme Court today threw out a 63-year-old law designed to restrain the influence of big business and unions on elections, 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 year's crucial midterm elections.

By a 5-4 vote, the court overturned two of its own decisions as well as the decades-old law that said companies and labor 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.

Critics of the stricter limits have argued that they amount to an unconstitutional restraint of free speech, and the court majority 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.

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."

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 midterm congressional elections.