Washington ? The Obama administration and its allies are going all out against the U.S. Chamber of Commerce and GOP-leaning groups, accusing them of using foreign money to help finance political ads. Trouble is, they’re providing no evidence.

The mere idea, lack of proof aside, is part of a Democratic message that tries to tie Republicans to foreign interests and to jobs shifted overseas. On Monday, the liberal group MoveOn.org began airing an ad in Illinois against Senate candidate Mark Kirk using his support from the chamber to link him to foreign corporations that, in the ad’s words, “threaten American jobs.”

And in Pennsylvania, Vice President Joe Biden kept up the attack on the chamber and other groups that are spending millions of dollars in political ads against Democrats without having to reveal their contributors.

“I challenge the Chamber of Commerce to tell us how much of the money they’re investing is from foreign sources,” Biden said during a fundraiser for Democratic Rep. Chris Carney in Scranton. “I challenge them, if I’m wrong I will stand corrected. But show me, show me.”

Republicans suggest it’s more about digging up votes than digging up answers, a sign of desperation by Democrats worried that they’re about to lose control of the House. And the chamber, which has already spent more than $20 million this year in campaigns mostly aimed at Democrats, is responding with an equally combative tone.

“We accept the vice president’s challenge here and now, and are happy to provide our answer: Zero. As in, ‘Not a single cent,'” said Tom Collamore, a senior vice president at the chamber. “We hope this clears it up, and hope the vice president keeps his word and stands corrected.”

Bruce Josten, the chamber’s top lobbyist, was even more pointed: “We are seeing an attempt to demonize specific groups and distract Americans from a failed economic agenda,” he said in a statement.

Using foreign money to pay for political activity is illegal, and the Chamber of Commerce says the minimal amount of money it receives from overseas is carefully segregated from political spending. The chamber says its 115 foreign business councils, known as “AmChams,” pay a total of about $100,000.

“There is no evidence at all that the chamber has done anything illegally,” said Richard L. Hasen, an expert on election law at Loyola Law School in Los Angeles. But he also noted that the potential for impropriety exists with any organization that accepts funds that would be illegal if spent on politics.

To be sure, money that is segregated for other purposes can free legal funds for political uses.

But the chamber is hardly alone. A number of labor unions and advocacy groups that participate in politics have foreign affiliates and overseas donors. By law, these groups must make sure no foreign funds are used to advocate for or against political candidates. What’s more, foreign companies with United States divisions can create political action committees that accept donations from their U.S. employees.