O'Donnell's attorney Cleta Mitchell dismissed the charge as outlandish and described CREW as a "left-wing front group" funded by the liberal financier Soros.
"If Melanie Sloan wants to deny that, you tell Melanie Sloan to reveal her donors," Mitchell told the Christian Science Monitor, referencing CREW's executive director. "She is not a neutral arbiter of ethics." Mitchell did not immediately return a call requesting comment.
O'Donnell chimed in on the group's political leanings as well telling CNN last night: "Why are you listening to a liberal organization in the first place?"
Sloan, a former federal prosecutor, came out swinging against O'Donnell in a press release.
"Christine O'Donnell is clearly a criminal, and like any crook she should be prosecuted," said Sloan. "Ms. O'Donnell has spent years embezzling money from her campaign to cover her personal expenses. Republicans and Democrats don't agree on much these days, but both sides should agree on one point: thieves belong in jail not the United States Senate."
Sloan wasn't available for an interview Tuesday afternoon, but a CREW representative defended the organization's non-partisan origins.
"We have a clear record as a non-partisan organization. I'm aware these allegations are out there -- you know has always been an equal opportunity antagonist. CREW accepts money from ordinary citizens who support our cause and we're supported by people on both sides of the aisle for actions that CREW takes," Adam Ratliff of CREW told TPM.
Ratliff pointed out that CREW had gone after Charlie Rangel and Maxine Waters, and said the O'Donnell filing was not partisan at all.
The charges against O'Donnell are partially based an affidavit from her former campaign aide David Keegan.
Soros is a member of the Democracy Alliance, which has provided money to CREW, and his Open Society Institute has given money to the watchdog group as well, according to Media Matters.