Interest groups are spending five times as much on the 2010 congressional elections as they did on the last midterms, and they are more secretive than ever about where that money is coming from.

The $80 million spent so far by groups outside the Democratic and Republican parties dwarfs the $16 million spent at this point for the 2006 midterms. In that election, the vast majority of money - more than 90 percent - was disclosed along with donors' identities. This year, that figure has fallen to less than half of the total, according to data analyzed by The Washington Post.

The trends amount to a spending frenzy conducted largely in the shadows.

The bulk of the money is being spent by conservatives, who have swamped their Democratic-aligned competition by 7 to 1 in recent weeks. The wave of spending is made possible in part by a series of Supreme Court rulings unleashing the ability of corporations and interest groups to spend money on politics. Conservative operatives also say they are riding the support of donors upset with Democratic policies they perceive as anti-business.

"The outside group spending is primarily being driven by the political climate," said Anthony Corrado, a professor of government at Colby College who studies campaign finance. "Organized groups are looking at great opportunity, and therefore there's great interest to spend money to influence the election. You've got the possibility of a change in the control of Congress."

The increase in conservative spending has come both from established groups and from groups only a few months old. On the left, major labor groups such as the Service Employees International Union have also ratcheted up their expenditures compared with 2006 but are unable to keep up with groups on the right.

One of the biggest spenders nationwide is a little-known Iowa group called the American Future Fund, which has spent $7 million on behalf of Republicans in more than two dozen House and Senate races. Donors for the group's ad campaign have not been disclosed in records the group has filed with the Federal Election Commission.

The group recently entered a previously sleepy race in its home state of Iowa, announcing that it would devote up to $800,000 to campaign against Democratic Rep. Bruce Braley of Waterloo. The campaign kicked off with a commercial alleging that Braley "supports building a mosque at Ground Zero." Braley denies supporting construction of the proposed Islamic cultural center near the World Trade Center site, saying it's a zoning issue for New Yorkers to decide.

The ad, part of a nationwide campaign of similar mosque-themed spots, is the brainchild of Larry McCarthy, a media strategist who gained renown for creating the racially tinged "Willie Horton" commercials against Democratic presidential candidate Michael S. Dukakis in 1988.

"Folks across America should be worried about these anonymous groups that go into an election and try to buy a favorable result," said Braley spokeswoman Caitlin Legacki. "People have no idea where the money came from. It's difficult to take recourse."

Interest groups spending large amounts on the election are prohibited by law from talking to candidates about their strategy.

Ben Lange, Braley's GOP challenger, denies any connection to the American Future Fund's attacks. "We have no interaction with this group," said Cody Brown, spokesman for Lange. "We're not so much concerned with what these outside groups are doing. We want to have an honest, focused debate on the issues."