The 5-4 ruling, in Citizens United v. Federal Election Commission, will radically alter the political landscape, supporters and detractors alike said Thursday, permitting unlimited new forays by the country's biggest companies, unions and nonprofit organizations into races for the U.S. Congress and the presidency.

While some groups hailed what they called a recognition of corporations' rights to free political speech, activists and politicians who have fought to rid U.S. politics of the influence of special-interest money said the results of the ruling will be devastating and will bolster the already significant influence of wealthy organizations to the detriment of individual voters.

"It's near-catastrophic for the republic," said U.S. Rep. John Larson, D-1st District. "As though Washington isn't already awash with special-interest money. It's just staggering, the corrosive nature of this and what it will do."

"This is the most radical and destructive campaign finance decision in the history of the Supreme Court," said Fred Wertheimer, president of Democracy 21, the nonpartisan group that seeks to blunt the influence of moneyed interests in politics.

"This court has said it's the constitutional right of a corporation to spend as much money as it wants to influence an election," Wertheimer said. "Insurance companies, drug companies, banks, et al., will each be free ... to literally run campaigns, direct mail, phone banks, TV campaigns, and thereby to buy influence over the members' positions."

The greatest effect may not even be during election season itself, Wertheimer and others warned. In the process of lawmaking itself, the threat of a big punitive challenge from corporate or labor interests could weaken the resolve of members of Congress to take tough votes or advocate controversial positions, they said.

"You will have corporations influencing the psyche of Congress without even having to spend the money," said Wertheimer. "The fear factor this will create without even the reality of the money flooding the system is potentially infinite."

But some conservative lobbying groups cheered the decision, embracing the court's view that corporations enjoy the same right to political speech as individuals under the First Amendment, and that the electioneering restrictions unconstitutionally infringed those rights.

"Under this ruling, corporate entity restrictions on political campaigning have thankfully been overturned, as they have a chilling effect on legitimate political speech protected by the First Amendment," said Bill Wilson, the president of the Americans for Limited Government, which advocates free-market solutions to policy problems, in a written statement.

The case began with the attempt by the nonprofit Citizens United to use pay-per-view cable to air a 90-minute video sharply critical of then-Sen. Hillary Clinton during the 2008 Democratic presidential primary. In its ruling, the court found the video to be essentially an extended negative campaign commercial urging Clinton's defeat, but the majority ruled that the FEC's finding that Citizens United could not pay to air it during the campaign violated the corporation's right to political speech.

After a rare re-argument of the case, many campaign finance activists said they had feared that the court's conservative wing, along with Justice Anthony Kennedy as a swing vote, would issue just the sort of broad ruling it did on Thursday, not simply ruling in favor of Citizens United but striking down the overriding prohibition on use of corporate money to influence elections.

The court's decision leaves in place requirements that corporations disclose major spending on electioneering advertisement campaigns, as well as the continued prohibition on corporations and unions giving contributions from their general treasury funds to political candidates. Such contributions would continue to be limited and must be made through political action committees separate from corporate treasuries, as under current law.

Observers noted, however, that the disclosure requirements could prove to be ineffective since corporations or unions not wishing to work expressly against a certain candidate could funnel money to a different company or nonprofit group in order to obscure where the funds had originated. (The U.S. Chamber of Commerce, which advocates mainly against Democratic and liberal causes, has already been accused of performing a similar service for major corporate clients over the course of the 2009 health care debate.)

But companies may now expressly advocate the election or defeat of individual candidates with unlimited amounts of direct spending on advertising and political messaging, the court said, a prospect that many Democratic political officials said would invite corruption and thwart the political aims of those without the means to spend heavily.

The decision "returns us to the days of the robber barons, where Wall Street and corporate influence will be even more exaggerated than it has already been," said Bob Edgar, a former congressman who now serves as the president of Common Cause, a major advocate of campaign finance reform at the federal and state levels.

"As a former member of Congress, I know what it takes to run for public office and I know the influence of money," he said. "In every congressional district across the country, in Senate races, even in the presidential race, money is going to go even further."

Larson and U.S. Sen. Dick Durbin, D-Ill., are co-sponsors of a bill that would provide for public financing of federal elections in exchange for contribution and spending limits, a plan supporters believe will help blunt the impact of the court's ruling. Larson said Thursday he would push for a vote in the House in the bill in the coming months.

At the state level, reaction was similarly gloomy.

"It's a terrible ruling, a breathtaking display of right-wing judicial activism at its worst," said Karen Hobert Flynn, the vice president for state operations of Common Cause Connecticut. "It went leaps and bounds beyond the facts of the case, and gutted decades of federal law in the process."

The reinvigoration of corporations as political actors could trigger a "new Gilded Age," said Vincent Moscardelli, an assistant professor in the University of Connecticut's department of political science.

"This will probably further erode citizens' faith in the democratic process," Moscardelli said, "and further support citizens' belief that elections are dominated by narrow moneyed interests. It could add fuel to the fires that drive polarization among political elites, because now you as a candidate are going to in many ways be more beholden, or at least more sensitive, to groups that have money and influence and can directly fight your re-election.

"The short-term implications, I think, are ominous for Democrats in 2010. For Democrats, what you've essentially done is turned the clock back to a time when private money from sources like this mattered more. And that was a regime in which Republicans did very well."

The Supreme Court ruling does not affect the current federal appeals court proceedings on Connecticut's campaign finance laws, Attorney General Richard Blumenthal said in a statement. But Hobert Flynn and others said they believed that state court challenges to contribution limits and spending restrictions could come in the wake of the ruling, now that it had established a federal precedent.

"This immoral decision makes an untenable system even worse," said Nick Nyhart, president of Public Campaign, a reform lobbying group.