Pressure has been mounting on the U.S. Chamber of Commerce since a ThinkProgress investigation published Tuesday revealed that the right-wing business group may be using donations from foreign corporations to bankroll its general account -- the same account that is being used to launch an unprecedented $75 million attack ad campaign against progressive candidates. Sen. Al Franken (D-MN) and others have called for an FEC investigation into the Chamber, and numerous media outlets have pressed the group for answers. Meanwhile, the Chamber continues to stonewall. Today, while stumping for Maryland Gov. Martin O'Malley (D), President Obama referenced the investigation, warning that "groups that receive foreign money" are a "threat to our democracy."