(CNN) - The independent GOP group American Crossroads put up two billboard ads this week, slamming Democratic Sen. Claire McCaskill of Missouri for a private jet scandal earlier this year.
Posted outside airports in St. Louis and Springfield, the signs read: "Sky-high airfares got you down? Senator Claire McCaskill: Flies in her own jet – and you pay for it!"
In March, McCaskill, a former Missouri state auditor, admitted she failed to pay nearly $300,000 in personal property taxes over the previous four years for the partial ownership she and her husband had 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 would sell the plane.
She also acknowledged that after a three-day trip through Missouri in May 2007, her Senate office improperly used taxpayer dollars for flights to two political events.
When she admitted the transaction, McCaskill said her staff had already repaid the government for the trip. In total, McCaskill reimbursed the government for $88,000 for 89 flights.
With McCaskill up for re-election in 2012, American Crossroads is bringing the issue back to light with the billboards and other ads.
One radio ad features a fake detective investigating McCaskill, using the senator's real voice for answers to his questions.
"I want to be held accountable," McCaskill said.
"I'm sure you will be, ma'am. Taking 80 grand from the federal government for flights on your own jet. It doesn't look good," the detective said. "But that's not why I'm here."
"I don't know what else I can do," McCaskill said in a clip pulled from a town hall meeting with a confrontational audience. "If you want me to go home…"
"That's why I'm here," the detective said.
The ad ends pointing listeners to a website the group created, called "Truth about Claire."
Totaling more than $75,000, the radio ad will run statewide for one week.
- CNN's Ted Barrett, Matt Hoye and Xuan Thai contributed to this report.