Washington (CNN) -- Groups spending millions on campaign ads against Democrats without having to reveal their donors were supplanting the Republican National Committee and threatening American democracy, White House Press Secretary Robert Gibbs said Tuesday.
"They're going to impact this election ... and nobody knows who they are," Gibbs said. "I think that's a threat to the democracy that we have in this country."
The comments to reporters showed the intention of the White House to keep up its attacks on what it calls anonymous campaign spending for Republicans in the final three weeks before the November 2 congressional elections.
Democrats are expected to lose seats, and perhaps their majority in the House, in the voting, and President Barack Obama has lamented a barrage of television anti-Democrat ads across the country by groups that don't have to disclose the identities of donors.
Obama mentioned the issue Tuesday night at a campaign event intended to rally young voters, saying the groups putting up the ads had a First Amendment right to do so, but it would be better if everyone had to disclose the donors.
"What's happening in this election is unprecedented," he said, citing "the ability of special interests to mobilize millions of dollars from donors who are undisclosed, to run negative ads at levels that are outspending, in some cases, the candidates or the parties."
The law can't be changed for the upcoming election, Obama said, adding that he hoped the Supreme Court sees what is happening and realizes "this is really hijacking our democracy; this is not a healthy thing."
Both Gibbs and Obama referred to the possibility that foreign money might be funding some of the attack ads against Democrats, with Gibbs specifically mentioning that the U.S. Chamber of Commerce -- a major donor to ads favoring Republicans -- gets some funding from overseas affiliates. Media reports and fact-checking web sites have questioned the claim, but Gibbs said the only way to know is for the Chamber to reveal its donors.
In a letter Tuesday to the U.S Chamber Board of Directors and federation partners, the group's president, Tom Donohue, denied that any foreign money goes toward the group's political donations.
"The Chamber does not use any foreign money to fund voter education activities -- period," Donohue said in the letter. "We have strict financial controls in place to ensure this."
He criticized Obama and Democratic leaders for what he called a "smear campaign" to try to salvage the upcoming elections.
"They hope that by demonizing those who oppose their failed policies, they can fire up their dispirited and disappointed base and silence our voice," the letter said.
Previously, Obama has used campaign speeches to cite groups associated with GOP strategist Karl Rove as an example of the problem.
One of the groups, Crossroads GPS, has spent millions on political advertising this election season but does not have to reveal the names of their donors to the Federal Elections Commission because it is structured as a tax-exempt "social welfare" 501(c)(4) organization.
Rove hit back Tuesday, accusing Obama of hypocrisy for challenging the group after accepting donations in the past from similar groups that support Democrats.
"For the president to say a legal and lawful action by private groups, which has been upheld by the Supreme Court of the United States, when he himself is relying on political groups. ... the president is being hypocritical about this," Rove said on ABC's "Good Morning America."
When reporters later noted to Gibbs that the groups he and Obama criticized were operating legally, Gibbs said "there's no reason to back off asking for the disclosure" of who is providing the money.
"The biggest point in this is disclosure. The biggest point is knowing what's out there," Gibbs said. He offered a solution to the political debate: "Simply disclose the identities of who funds all these ads."
"Simply tell us who you are," he said, later adding: "They are supplanting the entire effort of a national party to the tune of $150 million, $160 million, $180 million in a political campaign. And you don't have the slightest idea who they are. You don't know who they represent. You don't know what their political agenda is."
CNN's Alexander Mooney contributed to this story.