The president of the U.S. Chamber of Commerce said the business group would "ramp up" its work on behalf of candidates in the closing weeks of the election.

Chamber President Thomas Donohue wrote the group's board of directors to promise not to shrink in the face of attacks by President Obama and Democrats.

"[F]or the next three weeks leading up to Election Day you will see us ramp up efforts to educate voters about the positions of candidates of both parties who are committed to free enterprise and economic growth," Donohue wrote Tuesday in the letter, the text of which was posted by the New York Times.

The Chamber has found itself under siege, along with several other groups that have spent big this election cycle largely on behalf of Republicans, from the White House, congressional Democrats and a series of liberal groups who assert the Chamber might be spending foreign money on their political efforts.

"This is patently untrue," Donohue wrote of those allegations. "Let me be clear. The Chamber does not use any foreign money to fund voter education activities — period."

Still, some Democrats said their attacks on the Chamber and groups like American Crossroads, which is affiliated with GOP strategists Karl Rove and Ed Gillespie, who were both advisers in President George W. Bush's administration, is getting their party political traction.

"I think it is having resonance," said Rep. Chris Van Hollen (Md.), the leader of House Democrats' campaign efforts, about the attacks on the Chamber during an appearance on MSNBC.

"I think that people are understanding that there's this very important nexus between the special interests who are spending these millions of dollars and an agenda that doesn't serve the interests of the American people," Van Hollen added, calling his party's attacks a "wake-up call" for voters.

But Donohue said the increased pressure wouldn't keep the Chamber from its efforts on behalf of a number of Republican (and some Democratic) candidates over the next 21 days.

"We will not be deterred from full and vigorous participation in the political process," he said. "And, we will continue to successfully protect the rights and privacy of our members as they exercise their Constitutionally-guaranteed freedom of association in supporting the Chamber’s programs and activities."