"She goes to Washington, D.C., it's a little bit like one of those dogs, you know, 'fetch,'" Akin can be heard saying at a fundraiser in Springfield Saturday.
"She goes to Washington, D.C., and gets all of these taxes and red tape and bureaucracy and executive orders and agencies, and she brings all of this stuff and dumps it on us in Missouri."
The audio tape was leaked to PoliticMo, The Kansas City Star reported.
Akin's campaign staff didn't get back to the newspaper, which sought to ask the congressman about the taped remarks. The senator's campaign staff declined to comment, the newspaper said.
Akin, whose attempt to unseat McCaskill was dealt a blow in August when he said rape victims have a biological ability to avoid pregnancy, trails her in the latest voter survey by Public Policy Polling 46 percent to 40 percent. Libertarian Jonathan Dine had a 6 percent share.