IRS documents reviewed by The Associated Press show the group - Savior's Alliance for Lifting the Truth - is on a list of thousands of small organizations threatened with revocation of nonprofit status because they have not filed returns for the past three years.
O'Donnell, a tea party-backed candidate who stunned the GOP by winning the Senate nomination, founded the group in 1996 and has run it ever since. It is a pro-abstinence outreach organization aimed at young Christians.
Cleta Mitchell, an attorney representing the O'Donnell campaign, said the oversight is common among small charities with few resources and staff.
"It's not any big deal. I'm dealing with this for all kinds of clients right now," she said. "There are thousands of nonprofits doing this. Everyone is scurrying around."
Mitchell said she had no information about the Savior's Alliance's finances.
A 2006 law required nonprofit organizations with receipts of less than $25,000 to file tax returns for the first time starting in 2007. The IRS told charities that if they failed to file for three years, they would lose their tax-exempt status.
The original deadline was in May, but the deadline was extended until Oct. 15 because about 325,000 organizations had failed to comply. The IRS says many of those charities may no longer exist. In the case of Savior's Alliance, O'Donnell listed herself as its current president on the financial disclosure documents she filed with the U.S. Senate in July.
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The alliance did file a tax return in 2006 listing contributions of $2,000 and expenses of $1,973.
The tax issues are the latest revelation about O'Donnell's background.
She has made incorrect or misleading statements about the vote outcome in her 2008 Senate bid, her education and has rarely held a full-time job. She also has condemned masturbation and said she dabbled in witchcraft as a teenager.
The formerly little-known candidate who is in her third Senate bid defeated Rep. Mike Castle for the Republican nomination on Sept. 14. She now faces Democrat Chris Coons in the November general election.