ok, put yourself in Santorum’s shoes for a minute:
You are serving in the US Senate, with John McCain. You’ve gotten to witness first hand McCain’s actions, leadership, etc (McCain/Feingold, Gang of 14, McCain/Kennedy, Opposing the Bush Tax Cuts). You’ve really, really, grown to dislike McCain over your tenure in the Senate and agree with Thad Cochrane when he says “the thought of him being President sends shivers up my spine”.
Now witnessing that first hand, are you going to sit by and let McCain grab the nomination or are you going to do something about it.
That’s what and why Santorum did and said what he did in 2008.
I would not have endorsed neither McCain nor Romney.
Endorsements have consequences... as we can see today.