Mitt Romney's presidential campaign and an allied independent group have launched a TV ad blitz aimed at undercutting his GOP rivals.
Together, Romney's campaign and the independent group are spending about $2.6 million on television and radio ads.
Romney's ads are running only in Michigan, which votes Feb. 28.
The independent group is running ads in several states, including a spot critical of Rick Santorum.
Santorum has emerged in the past week as Romney's chief rival for the GOP nomination after sweeping contests in Colorado, Minnesota and Missouri.
Romney grew up in Michigan and his father was governor. Santorum is making a play for the state, where his blue-collar roots and pro-manufacturing message appear to be resonating. But he has spent significantly less on advertising.