Washington (CNN) - Sen. Claire McCaskill, D-Missouri, acknowledged Monday that she failed to pay nearly $300,000 in personal property taxes owed over the last four years for the partial ownership she and her husband have in a private plane.
McCaskill, a vocal supporter of reform and transparency in the Senate, described the failure to pay taxes as unintentional and said she will sell the plane.
"I take full responsibility for the mistake," she said in a conference call with reporters.
She said she is "disappointed in myself" and "sick to my stomach" over the matter.
The first-term senator, who is likely to face one of the toughest re-election battles in the country next year, wrote a check to the St. Louis County Tax Assessor Tuesday for $287,283.41. McCaskill said she and her husband did pay sales taxes on the Pilatus Turboprop, which they own with other investors, and are prepared to pay more to the county if it is determined additional taxes and fees are owed.
Republican campaign officials in Washington seized on her lapse.
"Can Missouri voters even believe anything Senator McCaskill says anymore?" Rob Jesmer, of the National Republican Senatorial Committee asked in a statement. "Now, millionaire Claire McCaskill wants to simply write yet another big check and hope people won't ask anymore questions."
McCaskill's acknowledgement comes shortly after Politico reported that she used tens of thousand of dollars from her Senate office budget to cover the costs of chartering the plane for Senate business. After the story broke, McCaskill repaid the U.S. Treasury $88,000 for 89 flights.
McCaskill said the disclosure had created a "perception" problem and the "appearance of impropriety." She sought a full accounting of her use of the plane which revealed the failure to pay taxes.
In a conference call with reporters, McCaskill, a former Missouri state auditor, said the review also found that after a three-day trip through Missouri in May 2007, her Senate office improperly used taxpayer dollars for flights to two political events. She said Tuesday the government had already been repaid for that travel as part of the $88,000 reimbursement.
McCaskill said the plane has "just been more trouble that it's worth."
"I have convinced my husband to sell the damn plane," she said. "He has hired a broker and I can tell you that I will not be setting foot in that damn plane again."