Republican Senate candidate Christine O'Donnell used more than $20,000 in campaign funds to pay her rent and other personal expenses, according to a complaint filed Monday with the Federal Elections Commission.

Backed by tea party activists, O'Donnell upset longtime Rep. Mike Castle in Delaware's GOP Senate primary and will face Democrat Chris Coons in the battle for Vice President Joe Biden's former seat.

The complaint filed by Citizens for Responsibility and Ethics in Washington, a nonpartisan watchdog group, is the latest in a series of allegations of financial irregularities involving O'Donnell, a conservative Christian activist and frequent candidate who has not had a steady job in years.

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O'Donnell dismissed the allegations, saying they were the work of opponents trying to derail her campaign.

"The momentum surrounding this campaign is obvious; that's why they're creating baseless accusations to try to stop it," she told reporters after a candidate forum Monday night in Middletown, Del.

"I am confident that we have been ethical," O'Donnell said. "We have not, I personally have not, misused the campaign funds. We have our FEC lawyer, a great attorney, answering those charges if it ever goes anywhere."

The attorney, Cleta Mitchell, said the O'Donnell campaign had not been served with the complaint even though it was posted on CREW's website.

"These are outlandish allegations," said Mitchell, who described CREW as a "left-wing front group" funded by liberal financier George Soros.

"If Melanie Sloan wants to deny that, you tell Melanie Sloan to reveal her donors," Mitchell added, referring to CREW's executive director. "She is not a neutral arbiter of ethics."

In a March interview with The News Journal of Wilmington, O'Donnell acknowledged using campaign funds to pay half the rent at her current town home and said it was legal because of the home's dual purpose as a campaign headquarters.

O'Donnell essentially used her campaign's bank card as her personal ATM, CREW executive director Melanie Sloan said, and the improper spending likely would have gone unnoticed if not for her surprise victory in last week's primary.

"She never thought anybody would look at her campaign finances very closely because she was not considered a serious candidate. She thought she could get away with this," Sloan said. "Now we're talking about a candidate for U.S. Senate. That's pretty serious, and there's no reason that Ms. O'Donnell shouldn't have to follow the same rules as everybody else."

The Democratic Senatorial Campaign Committee on Monday released an ad that attacks O'Donnell's financial history.

Besides the FEC complaint, CREW sent a letter to Delaware U.S. Attorney David C. Weiss asking for a criminal investigation of O'Donnell. Through his secretary, Weiss acknowledged receiving the letter but declined to comment further.

The complaint is based largely on a sworn statement by David Keegan, a former campaign finance consultant for O'Donnell. Keegan's nephew, Brent Vasher, purchased O'Donnell's Wilmington, Del., home for her in 2008 because she was facing foreclosure, according to Keegan's affidavit.

Vasher began charging O'Donnell $750 a month in rent in January 2009, and in March and April of that year, she paid the rent from her campaign treasury, according to the complaint. Documents filed with the FEC show two $750 payments to Vasher during the months in question. The payments are listed as reimbursement for expenses.

CREW also alleges that O'Donnell spent campaign money on gas for personal travel, meals and a bowling outing. This year, she continued to use the campaign treasury to pay rent at her new residence in Greenville, Del., which doubles as her campaign headquarters, as well as utility and wireless phone bills. The questionable 2010 expenses total more than $20,000.

O'Donnell has also been subject to an IRS tax lien and has been accused of leaving a trail of unpaid bills.

Associated Press correspondent Randall Chase in Middletown, Del., contributed to this report.

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