JEFFERSON CITY, Mo. -- It's not the most unflattering of photos, as far as negative TV ads go. But Democratic U.S. Senate candidate Robin Carnahan has asked a conservative interest group to pull its ad showing a photo of her in a wig she wore while being treated for cancer.

Carnahan's campaign has also asked American Crossroads, a group launched under the direction of former George W. Bush political operative Karl Rove, for an apology.

"This is just another disgusting attack from out-of-state corporate special interest groups who will say and do anything with no regard for facts or tact," Carnahan spokesman Rachel Barinbaum said Wednesday.

Jonathan Collegio, a spokesman for American Crossroads, declined to comment about the matter.

American Crossroads has run several ads against Carnahan and in support of her Republican rival, U.S. Rep. Roy Blunt.

Carnahan underwent treatment for breast cancer in 2006.

The commercial is not the first to use a photo of Carnahan in a wig she wore while ill. Ads by the U.S. Chamber of Commerce, which has backed many Republican candidates for national office, and the 7th District Congressional Republican Committee did earlier this year.

J.P. Fielder, spokesman for the U.S. Chamber of Commerce, didn't comment directly about the photo when asked. But he said in an e-mail Wednesday that "with less than three weeks left until election day, we're going to keep the debate focused on who has the best plan to create jobs in Missouri."

Gordon Kinne, chairman of the 7th District GOP committee, said he was not involved in putting together his group's ad, which aired before he took over leadership of the group. Kinne said he would have blocked the photo if he had known it was from when Carnahan was receiving treatment for cancer.

But "I would truly doubt it was intentional on our part," Kinne said. "It was probably just some footage they were able to gather from some place."