I base it on his history
Audio: Rick Santorum: If you're a conservative there's really only one place to go right now and that's Mitt Romney - 2008
That was as opposed to John McCain.
Good luck supporting the Senator from Arizona.
This should be an ad for Romney.
Mercy. Now, I know who he is truly for.
Mark Levin explained this - he also came out, last minute, for Mitt. According to Levin, at that point when it was just McLame and Romney, Romney was more conservative than McLame.