The U.S. Chamber of Commerce has more than doubled its spending on political attack ads since Tuesday, prompting criticism from liberal groups that question the origin of the funds.

The chamber reported spending $9.3 million on ads targeting Democrats in eight Senate and 22 House races since Tuesday. It had spent just less than $9 million this year through Monday.

Chamber President Tom Donohue has pledged to spend $75 million this election season as the business group tries to oust Members who voted for policies it opposes, such as the health care and financial regulatory overhauls.

The new ad buys bring the chamber’s television ad spending to just less than $20 million, still far short of the $75 million goal. The chamber has also contributed $4.6 million to 527s, most of them affiliated with Republicans.

The business group has also said it expects to run a large grass-roots operation, with mailers and phone banks, in the final weeks of the campaign.

The accelerated spending in recent days has drawn sharp protests from Democrats and liberal groups, who recently pounced on an accusation by Think Progress that the chamber was using foreign corporate dues to pay for ads.

One watchdog entity, Campaign Money Watch, issued a statement Thursday calling on candidates who are being helped by the new chamber ads to ask the business group about its financing for the spots.

“No candidate should sit quietly by and accept help from TV advertising that is furthering the special interest of the chamber,” said David Donnelly, director of Campaign Money Watch.

The chamber is not required to disclose the companies or individuals underwriting its political activity.

Chamber officials dismissed the attacks. “These accusations are a desperate attempt to silence those who support free enterprise and a diversion from their lack of progress on job creation,” said Tita Freeman, the chamber’s vice president for communications.

The largest buy in the recent batch of chamber ads is $1 million for spots against Rep. Paul Hodes (D-N.H.), who is running for a Senate seat against Kelly Ayotte (R). The chamber is also targeting Sen. Michael Bennet (D-Colo.), Sen. Russ Feingold (D-Wis.), Rep. Joe Sestak (D-Pa.), Robin Carnahan (D-Mo.), Alexi Giannoulias (D-Ill.), Richard Blumenthal (D-Conn.) and Florida Gov. Charlie Crist, who is running as an Independent. They are all running for Senate seats.

The chamber is also running ads against incumbent House Democrats in California, Colorado, Florida, Illinois, Kansas, Nevada, New Jersey, New Mexico, North Dakota, Ohio, Pennsylvania, Virginia and Wisconsin.