WASHINGTON — When the Supreme Court ruled that corporations had the right to political free speech, the decision set loose a tidal wave of campaign money that helped elect a new Congress in 2010 and now is reshaping the presidential race.

But the impact of the Citizens United decision has been as surprising and controversial as the ruling. Although the high court's 5-4 decision is best known for saying corporations may spend freely on campaign ads, the gusher of money pouring into this year's campaigns mostly hasn't involved corporate funds. And some practices that critics of the decision decry actually stem from a later case decided by a federal appellate court after Citizens United.

The rise of "super PACs," which may raise and spend unlimited amounts so long as they do so independently of a candidate, has allowed close aides to candidates to set up supposedly independent committees that have raised huge amounts, primarily from wealthy individuals. The PACs mostly have spent the money on negative, attack ads. That unlimited fundraising was set in motion by Citizens United but came to full flower after the appellate decision.

By design or happenstance, a two-track campaign-funding system has been created: One features small donors and strict regulation; the other exists for the very wealthy, who are largely freed from regulation.

Exasperated defenders of campaign-finance laws see the Citizens United decision as a historic blunder that has all but destroyed not only the 1940s limits on campaign spending by corporations and unions, but also post-Watergate reforms. They are asking the justices to reconsider the Citizens United ruling by taking up a Montana case that raises some of the same issues.

Fred Wertheimer, a champion of campaign-finance laws, says the decision has "fundamentally undermined our democracy and is taking the nation back to the system of 'legalized bribery' that existed in the robber baron and Watergate eras."

The Supreme Court will meet behind closed doors Thursday to discuss the Montana case. The five justices who supported Citizens United, led by Justice Anthony Kennedy, are not likely to agree with the critics. But they may be concerned over how political spending has shifted away from candidates and political parties and toward outside groups.

Political-action committees were common before 2010. They allowed like-minded people — including a company's employees — to contribute as much as $5,000 each to candidates or campaigns. But in March 2010, two months after the Citizens United ruling, the contribution lid was lifted in a related case, SpeechNow.org vs. FEC. The U.S. Circuit Court of Appeals for the District of Columbia, citing the 5-4 opinion, reasoned that, because the First Amendment guaranteed unrestricted "independent" political spending, PACs should have the right to collect unlimited sums, if they too were independent.

Thus, the parallel system was born. Congress had set limits on individual contributions after Watergate, and they remain in effect. A person who wants to contribute to the campaigns of President Obama or Republican challenger Mitt Romney may give no more than $5,000 this election cycle. But those who have $1 million to spend can send money to a super PAC supporting Obama or Romney. Restore Our Future, a super PAC that supports Romney, has at least 16 donors who have given more than $1 million.

"The real impact of Citizens United," Columbia law professor Richard Briffault said, has been to legalize "the unlimited use of private wealth in elections. You haven't seen nearly as much business or corporate money as people expected. Most corporations are not eager to be involved in an obvious way."

Super PACs must disclose donors, but those who wish to maintain anonymity can do so through the nonprofits and trade associations that do not disclose donors. "More than $120 million in anonymous funds was spent to influence the 2010 elections," the Campaign Legal Center reported. That number is expected to be far higher in 2012, the group said.

The Citizens United issue now returns to the high court after the Montana Supreme Court, citing a history of "copper kings" who bribed legislators, refused to strike down a state ban on election spending by corporations.

Indiana attorney James Bopp, a major player in the Citizens United case, appealed and urged the justices to straighten out the recalcitrant state justices. Defenders of campaign-finance laws, including Sen. John McCain, R-Ariz., launched a separate attack on what they say are errors and "faulty assumptions" in the Citizens United opinion.

While the high court turns down 99 percent of appeals, no one expects the Montana appeal to be denied.

The justices could write a summary opinion explaining why Citizens United was right — or hear the case in the fall and reconsider whether it was a mistake.