RENEE MONTAGNE, HOST:
Let's talk more about where Rick Santorum stands and about his rivals, with NPR's national political correspondent Mara Liasson. Good morning.
MARA LIASSON, BYLINE: Good morning, Renee.
MONTAGNE: Now, we just heard from Sonari that Rick Santorum has surged in Michigan, and that would be, of course, Mitt Romney's native state. And that is one that Romney really cannot afford to lose. So what is he doing to stop Santorum, as of today, and what's it looking like?