To apply that sort of uniform swing is idiotic. There's every reason to believe that Santorum would actually improve on McCain's numbers in most of the states that McCain won in 2008.
Most of the GOP collapse would be in the Obama states. Furthermore, Santorum would be free to do certain things to appeal to the middle that Romney can't. A Romney/Rice ticket is impossible because the base would never stand for it, but while they would grumble, a Santorum/Rice ticket would be acceptable to the base.
That said, the article didn't say Santorum would get the nod, but whoever Romney picks as his running mate, and that definitely will not be Santorum.
In the south, Santorum's numbers would probably rise, but there's no way in hell Santorum would do better outside that area, because while Santorums win would rally the base some (he's still fiscally more liberal than the party would probably like), it would rally the democrats FAR more.