The White House intensified its attacks Sunday on the powerful U.S. Chamber of Commerce for its alleged ties to foreign donors, part of an escalating Democratic effort to link Republican allies with corporate and overseas interests ahead of the November midterm elections.

The chamber adamantly denies that foreign funds are used in its U.S. election efforts, accusing Democrats of orchestrating a speculative smear campaign during a desperate political year.

President Obama, speaking at a rally in Philadelphia, said "the American people deserve to know who is trying to sway their elections" and raised the possibility that foreigners could be funding his opponents.

"You don't know," Obama said at the rally for Senate candidate Joe Sestak and other Democrats. "It could be the oil industry. It could even be foreign-owned corporations. You don't know because they don't have to disclose."

The remarks are part of a volley of recent attacks by Obama and other Democrats on alleged foreign influence within the Republican caucus, whether through support for outsourcing jobs by major U.S. corporations or through overseas money making its way into the coffers of GOP-leaning interest groups.

The comments also come as Democrats attempt to cope with an onslaught of independent political advertising aimed at bolstering Republicans, much of it fueled by donations that do not have to be revealed to the public. The spending has added to a political environment in which Democrats are in danger of losing control of both the House and Senate.

David Axelrod, a top Obama adviser, said on CBS's "Face the Nation" that secret political donations to the chamber and other groups pose "a threat to our democracy."

Axelrod also took the unusual step of calling on the chamber to release internal documents backing up its contention that foreign money is not being used to pay for U.S. political activities. Democrats have seized on a report by a liberal blog alleging that dues from chamber-affiliated business councils could be used in that way.

"If the chamber opens up its books and says, 'Here's where our political money's coming from,' then we'll know," Axelrod said. "But until they do that, all we have is their assertion."

The chamber has vehemently denied the allegations, characterizing them as part of a desperate strategy to stave off a GOP takeover of Congress. The business lobby has vowed to spend up to $75 million on the midterm elections, primarily in favor of Republicans.

Chamber senior vice president Tom Collamore called the Democratic attacks "a blatant attempt to avoid a serious discussion of Americans' top priority - creating jobs and growing the economy."

The Democratic National Committee began airing ads over the weekend attacking the chamber as "shills for big business" and claiming: "It appears they've even taken foreign money to spend on our elections." The ad also attacks Karl Rove, former Bush administration political adviser, and Ed Gillespie, former Republican National Committee chief, for their ties to American Crossroads, an independent group also spending big on election ads this year.