The ad claims McCaskill supported cuts to Medicare spending by supporting President Obama's healthcare legislation. It also accuses her of standing in the way of middle-class tax cuts.
Crossroads said the ad will air statewide for two weeks.
"Tell Claire, on Medicare and taxes, start voting in Washington the way you talk in Missouri," the narrator in the ad says.
Crossroads has been on the air in the race for months with hundreds of thousands of dollars of television and radio spots attacking the centrist senator. Republicans are eyeing her seat as one of their best pickup opportunities for 2012.
The Hill rates this race as a toss-up.
Watch the ad: