WASHINGTON  More than a dozen political groups have organized in recent weeks to spend large amounts of money on attack ads against House and Senate candidates as special interests on the left and right play a larger role in midterm races.

They include Florida Is Not For Sale, which has spent more than $245,000 on ads to influence the outcome of today's Democratic primary in Florida, and Americans for New Leadership, a Republican-affiliated group that has run ads against Senate Majority Leader Harry Reid, a Democrat.

Recent federal court rulings allow outside groups to collect unlimited corporate and union cash and spend it on independent ads that call for the election or defeat of candidates. Such activity had been illegal until earlier this year.

In many cases, the public will not know who has funded the ads until long after they have aired.

"This is the new order of political finance," says Sheila Krumholz, executive director of the Center for Responsive Politics, which tracks money in politics. Outside groups can "sling mud with hidden money."

Since June 1, at least 15 organizations have launched these operations to influence congressional races, according to filings with the Federal Election Commission.

Florida Is Not For Sale has run ads opposing billionaire Jeff Greene, who faces Rep. Kendrick Meek, D-Fla. Ben Pollara, who served as Hillary Rodham Clinton's top Florida fundraiser in the 2008 presidential race, is the group's treasurer. He said Meek supporters are trying to counter the millions Greene has spent of his own money.

Because of the ads' timing, Florida Is Not For Sale hasn't had to disclose the sources of most of its funding. Full reports won't be made until Oct. 15, when the next quarterly reports are due to the Federal Election Commission. Federal candidates also report donations on a quarterly basis.

"I believe all this stuff should be transparent, but I'm not the one who makes the law," Pollara says. "I'm going to play by the rules of the game."

Republicans also are capitalizing on the new rules to benefit allies. Americans for New Leadership, for instance, has spent more than $122,000 against Reid, who faces Republican Sharron Angle.

The group was formed to highlight Reid's record on the economy and taxes, says co-founder Jen Harrington, who worked in President George W. Bush's administration. She formed the group with Brent Husson, a former executive director of the Nevada Republican Party.

Americans for New Leadership also won't disclose donors until Oct. 15. "We have tens of thousands of donors," Harrington says. "We've tapped not just into local anger, but nationally as well."

Other groups spending heavily include American Crossroads, a Republican group created with the help of Bush's top political strategist Karl Rove. It recently spent $454,341 on advertising to help GOP Senate candidate Rob Portman in Ohio.

American Crossroads has publicly disclosed its donors in monthly reports. Its most recent filing for July activity shows it collected several large checks from corporations, including more than $841,000 from TRT Holdings, which owns Omni Hotels and Gold's Gym International.

A spinoff group called American Crossroads GPS is a non-profit that does not have to publicly disclose the sources of its funding. In recent weeks, it pumped $2 million into advertising to oppose Reid and Robin Carnahan, a Democrat running for the Senate in Missouri.