WASHINGTON -- Former Sen. Russ Feingold is taking the gloves off after the Supreme Court's summary reversal of a case that challenged its 2010 decision in Citizens United v. Federal Election Commission.

"This court had one fig leaf left after this one awful decision two years ago," said the former three-term Democratic senator from Wisconsin. The justices could claim "they were politically naive or didn't know what would happen when they overturned 100 years of law on corporate contributions," he said.

But with the court's decision on Monday, Feingold told HuffPost, that's gone: "They have shown themselves wantonly willing to undo our democracy."

Feingold, whose name is synonymous with campaign finance reform as co-author of the McCain-Feingold law, said he laughed bitterly when he heard the Supreme Court had slapped aside a Montana state court decision at odds with Citizens United. One of the most unpopular decisions in recent years, Citizens United opened the door for corporations and unions to spend freely in elections. The expansive language of the decision, written by Justice Anthony Kennedy, led a lower court to allow individuals to pool unlimited money through groups now known as super PACs. The two rulings have led to an explosion of independent spending in the 2010 and 2012 elections, with many of the donors hidden from public view.

Now the U.S. Supreme Court's decision to reverse the Montana Supreme Court's ruling in American Tradition Partnership v. Bullock, which had held the state's history of corporate corruption should exempt it from Citizens United, leads Feingold to his strongest attack on the Roberts Court to date.

"This is one of the great turning points, not only in campaign finance but also in our country's history," Feingold said. "I believe we're in a constitutional crisis."

Feingold, who heads an anti-Citizens United group called Progressives United, is particularly concerned about the Supreme Court's relationship to corporate power. "What they have clearly become is a partisan arm of corporate America. This is a real serious problem for our democracy," he said. "It's essentially a court that rules in one direction."

"Even if they do uphold the health care law, this court is no longer perceived as the independent arbiter of the law that the people expect them to be," Feingold added, referring to the highly anticipated decision coming Thursday.

According to a study by the Constitutional Accountability Center, the U.S. Chamber of Commerce, the chief business lobbying group in Washington, has a 68 percent win record in the Roberts Court on cases in which it has filed briefs, far higher than during similar recent periods of stability in the court's membership.

Linking the Roberts Court to previous courts that stymied progressive change -- the Supreme Court of the Gilded Age, the 1905 decision in Lochner v. New York and the court's initial resistance to the New Deal -- Feingold said, "This is one of those moments where the court thinks it's the legislature."

In a June article in the Stanford Law Review, Feingold discussed the rise of citizen democracy, powered by the Internet, from 2006 to 2008. Giving by small donors increased -- largely spurred by innovations in Barack Obama's 2008 presidential campaign -- and ordinary Americans found new avenues for organizing and pressuring politicians to listen to them.

Those changes are what sparked the Citizens United decision, Feingold told HuffPost. "The corporate interest in America saw the face of democracy, and so what they did was engineer this decision," he said. "They used it as an excuse to stop citizen democracy in this country."

But the people are fighting back. Feingold pointed to efforts across the country to pass resolutions calling for a constitutional amendment to overturn Citizens United and to reverse parts of the 1976 ruling in Buckley v. Valeo on which Citizens United was based. More than a half-dozen amendments have been offered in Congress to do the same.

In New York, advocates are pushing to replicate statewide the public campaign financing system instituted in New York City, which Feingold said is a workable model for other states and the country.

The reelection of President Obama is also important, said Feingold, as the next president might have the opportunity to shift the balance on the Supreme Court.

"All of these strategies are relevant to what I think is a significant movement in this country to overturn [Citizens United]," he said. "I believe it will be overturned."

In the meantime, Feingold agrees with his former campaign finance reform partner, Sen. John McCain (R-Ariz.), who said about Citizens United and the rise of super PACs, "I promise you there will be huge scandals."

Feingold goes one step farther: "There already is a scandal."