So far, Rick Santorum has scored a big win in Tennessee and at least a win (according to projections just coming in from the nets) in Oklahoma. The current numbers show a close race in Ohio. But the most recent exit poll from about an hour ago shows Romney winning by a single percentage point in Ohio. That means Romney probably wins there, more likely than not.
If he does it's not a great night because he lost in Oklahoma, Tennessee and Georgia. But if he loses Ohio, which does not look likely but definitely still possible, it's a catastrophic night for Mitt.