A coalition of liberal groups opposed to the U.S. Chamber of Commerce’s election spending took their cause to the big-business lobby’s front yard today.

In a Lafayette Square news conference that doubled as a mock 100th birthday party for the chamber, activists called on the group to disclose the corporations funding its political activities. The chamber has spent nearly $24 million trying to influence races this cycle, almost exclusively in support of Republicans, according to data compiled by the Center for Responsive Politics. The trade organization is not required by law to disclose any of its supporters.

Protesters represented a who’s who of the liberal-leaning watchdog groups, including U.S. PIRG, Public Citizen, the Main Street Alliance, Citizens for Responsibility and Ethics in Washington and former Vermont Gov. Howard Dean’s Democracy for America. The effort is part of a larger crusade against the influx of undisclosed and corporate money in the post-Citizens United era.

Speakers today highlighted the chamber’s activities in the race to replace retiring Maine Sen. Olympia Snowe (R), where the chamber is the largest unaffiliated outside spender. The business lobby has spent $1.34 million against Independent former Gov. Angus King, the frontrunner in the race, who has recently lost ground to his Republican opponent, Maine Secretary of State Charlie Summers. The National Republican Senatorial Committee has spent about the same amount while its Democratic counterpart has invested $1.09 million.

Melanie Collins, a Democrat and member of the Maine Small Business Coalition, said she is worried that more than $500,000 in attack ads recently run by the chamber could cost King the election by driving liberal voters to a relatively weak Democratic candidate, Cynthia Dill.

“A lot of people don’t really know what the chamber is,” said Collins, who runs a small child care company and traveled to Washington, D.C., to speak. “My sister thought it was a government agency.”

Outside groups have spent heavily in that race, injecting a total of $5.57 million, and plenty have lined up behind King, including Americans Elect, the third-party group that tried and failed to get an independent presidential candidate on the ballot this year. New York City Mayor Michael Bloomberg has contributed $500,000 through his newly formed super PAC to an ad campaign promoting King as an independent voice capable of breaking partisan gridlock in Washington.

The chamber reported $43 million in lobbying expenditures in the first half of 2012, a figure that includes money spent on advocacy and voter education in Washington, D.C., and around the country. Reports detailing expenditures for the third quarter are due Monday.

With the election just more than two weeks away, liberal-leaning groups have also taken aim at the National Federation of Independent Business, a small-business lobby that claims some 350,000 members. The Madison, Wis.-based Center for Media and Democracy is accusing the lobby of representing big-business groups and supporting a conservative agenda with a new website, NFIBexposed.org.

U.S. Chamber Watch, long a small but vigilant tracker of the chamber’s political activities, is now under the auspices of Public Citizen, one of the lead organizers of today’s event.

After the news conference, the activists, many of whom were sporting AFL-CIO and Communications Workers of America paraphernalia, delivered to the chamber a petition with 30,000 signatures calling on the group to disclose its corporate backers.

Correction: 2:12 p.m.

An earlier version of this article misstated the milestone of the mock birthday celebration for the U.S. Chamber of Commerce. It was a mock 100th birthday celebration.