American Crossroads, the outside campaign group with links to Karl Rove, launched a radio ad campaign Wednesday against Sen. Claire McCaskill, the centrist Democrat from Missouri who remains one of the GOP’s top targets as they try to take back the Senate in 2012.

Crossroads, which plans to spend $120 million on the 2012 elections, dropped $50,000 on the statewide media buy, a group spokesman said. The first ad is styled as a morning newscast, arguing that Ms. McCaskill’s messages to constituents differ from her voting record.