The US Supreme Court's 2010 decision to remove limits on political campaign financing dealt a heavy blow to American democracy, a documentary screened at the Sundance Film Festival claims.

"Citizen Koch" was brought to Sundance, which runs in this Utah mountain resort until January 27, by filmmakers Tia Lessin and Carl Deal.

Their new work is rooted in a highly controversial Supreme Court ruling that allowed large companies to set up political action committees (PACs) and provide unlimited campaign financing.

Dubbed "Super PACs," these committees cannot be formally linked to a candidate, but in effect support political campaigns through television ads.

One such committee, Americans for Prosperity, was founded and funded by two Koch brothers, owners of the conglomerate Koch Industries.

With their Super PAC, the two billionaires support Tea Party candidates, the ultra-conservative fringe of the Republican Party.

"There's been money in American elections for a very long time and certainly not only in the electoral system, but in the legislative level, the lobbying, for public policy," Lessin told AFP.

"But this is sort of a tsunami of money that we've never seen before. That's a real danger for democracy."

Lessin and Deal won the 2008 Sundance Grand Prix for "Trouble the Water," a documentary vividly describing the ravages of Hurricane Katrina in New Orleans.

"Citizen Koch" details the process that led to the creation of Super PACs, including the alleged conflict of interest surrounding two Supreme Court justices involved in the case.

The film also looks at the court ruling's direct application on the ground, particularly in the state of Wisconsin, where Republican Governor Scott Walker in 2011 faced a popular revolt after he decided to renegotiate collective bargaining agreements and limit the power of labor unions.

His opponents succeeded in calling a new gubernatorial election for June 2012, but Walker won this, with massive support from a Super PAC set up by the Koch brothers.

The group poured tens of millions of dollars into the campaign, but the Super PAC was not required to disclose the names of donors or how much they contributed.

"A lot of this happens in secret, behind closed doors," Deal said.

"Nobody knows how much money really pours into these elections."

The film follows three Republican voters -- a prison guard, a librarian and a nurse -- who were disgusted by the politics of Governor Walker and decided to vote Democrat because they did not like the Tea Party.

"It's not about Republicans versus Democrats," Lessin said. "It's about the voices of the one percent and the wealthiest among us versus the voices of working people, and whose voices gets heard."

For the filmmakers, this power of corporations and lobbying groups deprives Americans of candidates who do not have the financial muscle to enter the game.

But the system also puts political parties in the service of private interests.

According to Lessin, after again losing to Democrat Barack Obama, the Republican Party might think of changing course.

"They may think that they need, to get elected, to be more focused on the middle class," she said. "But who's funding them?