Common Cause has long been something of a nerd among the jocks. While other activists staged loud demonstrations and nervy stunts, the 40-year-old good-government group was more likely to hold a forum on filibuster reform or the vagaries of redistricting.

But suddenly Common Cause is manning the barricades, leading a rowdy campaign by liberal groups decrying the outsized role of big money in U.S. politics.

The main targets of the campaign are billionaires Charles and David Koch of the Koch Industries energy-and-paper conglomerate, who have spent tens of millions of dollars over the years on conservative issues and candidates. Liberal activists led by Common Cause staged noisy protests last month outside a private political gathering held by the brothers in Rancho Mirage, Calif.

The group also has spearheaded criticism of Supreme Court Justices Antonin Scalia and Clarence Thomas for making speeches at Koch-affiliated dinners. Common Cause revealed that Thomas had not reported his wife's employment by the Heritage Foundation and other conservative groups, as required by federal disclosure laws. Citing that, along with Virginia Thomas's links to groups that want the nation's new health-care law to be repealed, some House Democrats demanded Wednesday that Justice Thomas recuse himself from cases involving health-care reform.

Bob Edgar, a mild-mannered former Democratic congressman and Methodist minister who heads Common Cause, said the group's renewed activism hearkens back to its founding in 1970 as an anti-Vietnam War, pro-civil rights organization. The group recently named Clinton labor secretary Robert Reich as its chairman.

"It's really getting back to our roots," Edgar said. "We believe this is a very dangerous moment, where democracy is really at a crossroads."

He added: "We're going to continue to focus on the Supreme Court, particularly those who want to politicize the Supreme Court. And we're going to continue to use the Koch brothers as the poster children for a group of people who want to move our democracy toward a plutocracy."

Conservative critics, however, call the campaign shrill and unfair. Tim Phillips, president of Americans for Prosperity, which is chaired by David Koch, called the efforts a "fundraising ploy" and said Common Cause is hurting its mission by demonizing opponents.

"These guys had a decent reputation for ethics and civil politics, but they're clearly losing that reputation with these kinds of activities," Phillips said, noting preprinted signs at the California protest that read, "Koch Kills." "I can't speak to the motives as to why they're going after Charles and David in such a nasty, personal way."

Common Cause reported about $4.8 million in revenue in fiscal 2009, down from $6.6 million the year before, tax records show. The group's supporters have included the Open Society Institute, headed by liberal financier George Soros.

The Kochs and their company - the second-largest private firm in the United States - have been longtime financial supporters of conservative and libertarian causes, from the Cato Institute to the Federalist Society.

But liberal activists say they have been increasing their focus on the duo since last year's Supreme Court decision in Citizens United v. Federal Election Commission, which allows companies such as Koch Industries to spend unlimited amounts of money on elections, including in ways that are hidden from the public.