DENVER -- Public advocates anticipate that a U.S. Supreme ruling that allows big business to directly spend millions to support -- or oppose -- candidates will soon trigger a lawsuit challenging Colorado's campaign finance laws.

The Thursday high court decision could also unleash a flood of corporate and union spending on crucial midterm congressional election campaigns.

That means a bigger barrage of TV and radio political attack ads, said Jenny Flanagan, executive director of Colorado Common Cause.

"We expect to a see a lawsuit very soon attacking Colorado's campaign finance law, Amendment 27," said Flanagan, whose group authored the ballot measure passed by 67 percent of state voters in 2002.

Given the breadth of the Supreme Court's ruling in the Citizens United case, Flanagan said she believes a lawsuit against Amendment 27 is "likely to succeed on some -- but not all -- fronts."

In the split 5-4 ruling, the Supreme Court overturned two earlier decisions and threw out parts of a 63-year-old law that said companies and unions can be prohibited from using money from their treasuries to produce and run their own campaign ads. The decision threatens similar limits imposed by 24 states, including Colorado.

Critics of the stricter limits on corporate contributions 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. "We find no basis for the proposition that, in the context of political speech, the government may impose restrictions on certain disfavored speakers."

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."

Supporters of campaign finance reform said the ruling swept away a century of federal and state restrictions on corporate campaign-finance influence.

They fear it will open "flood gates" of corporate and union contributions in a political system where moneyed interests already drown out the voices of regular citizens.

"This is the Super Bowl of really, really bad decisions," said Common Cause national president Bob Edgar, a former congressman from Pennsylvania. "It has the potential to seriously corrupt and distort our democratic system in Congress and in the states if this decision is not corrected by Congress."

Fred Wertheimer, president of the nonprofit Democracy 21, predicts a "fear factor" for congressional lawmakers who will know that "every time they cast a vote they potentially face multimillion-dollar campaigns against them by corporations."

University of Denver business professor John Holcomb said the "hue and cry" is exaggerating the impact of the decision.

He noted that the Supreme Court ruling left in place a prohibition on direct contributions to candidates from corporations and unions. It also left alone the landmark McCain-Feingold bill's ban on corporate contributions to political parties.

The high court, Holcomb said, historically has supported restrictions on campaign contributions to candidates, reasoning that they're most likely to have a corrupting influence.

But, he said, the court has consistently ruled that constitutionally protected free speech isn't a right that can be equally apportioned among those with more or less money. Spending money to express one's political view is deemed freedom of expression.

"We don't limit the speech of one person to help the speech of another," he said.

Common Cause's Flanagan, however, said the Supreme Court ruling ignores the will of Colorado voters who have long supported restrictions on the corrupting influence of special-interest campaign contributions and expenditures.

"Maybe it's a victory of free speech for corporations, but I think it's a huge loss for the free speech of ordinary Americans," she said. "It's going to have huge impact."

The case began in 2008 when a conservative group, Citizens United, made a 90-minute movie that was sharply 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.

Federal courts said the movie looked and sounded like a long campaign ad, and therefore should be regulated like one. The high court rejected that view Thursday.