Two years ago this week, the Supreme Court set the political world on its head by ruling that corporations could spend unlimited money on elections, rolling back decades of legal restrictions.

An array of liberal-leaning activist groups are marking the anniversary by launching new efforts to overturn the decision, including calls for a potential constitutional amendment.

The 5 to 4 decision in Citizens United v. Federal Election Commission effectively laid the groundwork for super PACs, the new independent groups that have overwhelmed the Republican presidential race with millions of dollars in negative advertising over the past few weeks.

The decision, issued Jan. 21, 2010, outraged Democrats and many watchdog groups, but they’ve had little success in doing anything about it over the past two years. Activists are hoping to try again by focusing on the difficult task of passing a constitutional amendment, which would require ratification by three-quarters of the states.

“We’re already at a point where the public overwhelmingly opposes the decision,” said Robert Weissman, president of Public Citizen, a watchdog group helping to spearhead the efforts. “The goal is to build a grass-roots movement that will eventually be able to shape the debate.”

Public Citizen is teaming up with local activists to stage about 300 rallies and other events, most of them on Friday or Saturday, targeting multinational companies around the country. Many are being billed as “Occupying the Corporations” protests, inspired by the anti-Wall Street demonstrations that have taken hold in recent months.

Common Cause also launched its own pro-amendment project this week, dubbed Amend2012 and chaired by Clinton administration labor secretary Robert Reich. It includes a petition drive opposing the Supreme Court’s judgment that corporations have the same rights as people when it comes to political speech.

“It’s time to stop the unlimited flow of corrupting money into our elections,” the group’s petition reads. “To do that, we need a constitutional amendment to reverse Citizens United and declare that only people are people.”

On another front, the Corporate Reform Coalition, a group of institutional investors and others, is backing a petition drive urging the Securities and Exchange Commission to require corporations to disclose political spending to shareholders.

These and other such efforts essentially serve as an acknowledgment that previous attempts at reform after the Citizens United decision failed miserably. In 2010, the White House and Democrats in Congress tried to pass legislation requiring fuller disclosure of political spending by corporations, but the changes were blocked by Senate Republicans.

Advocates for stricter campaign finance laws are also disgruntled by the Federal Election Commission, which frequently deadlocks along party lines and has given super PACs wide latitude. Several watchdog groups are conducting a petition drive urging President Obama to fill vacant FEC seats. The seats remain occupied by previous commissioners for a lack of nominees.

“While the courts did plenty to create this mess . . . the FEC bears much responsibility for making a bad situation disastrous,” Meredith McGehee, policy director at Campaign Legal Center, said in a recent statement. “With super PACs running amok, the Republican presidential primary is exhibit A of a system out of control, and the FEC is complicit in this auctioning of the White House.”

Conservatives in large part take a much different tack, viewing Citizens United as just one step toward a less-regulated election system. Republican front-runner Mitt Romney, while complaining about some of the ads put out by super PACs, has argued in recent days that contribution and spending limits should be removed for candidates as well.

“I would like to get rid of the campaign finance laws that were put in place,” Romney said at a debate Monday night in Myrtle Beach, S.C. “ . . . Let people make contributions they want to make to campaigns, let campaigns then take responsibility for their own words and not have this strange situation we have.”

Amendments to the Constitution must first be proposed by a two-thirds supermajority vote of both the House and the Senate. (Two-thirds of state legislatures can also call for a national convention, but the approach has never been used.) Then 38 states would have to ratify the amendment, through either their legislatures or state-level conventions.

Amendments are notoriously difficult to launch, and even more difficult to get ratified. The 27th Amendment, which limits increases in congressional pay, was first proposed in 1789 — and not ratified until 203 years later, in 1992.

Weissman and others say they are under no illusions about how difficult such an effort would be.

“I think everyone understands that winning a constitutional amendment is an uphill fight and a long-term struggle,” he said.

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