Santorum is fading already. He whines and stammers and cannot fight back against Romney. Also, he has no new ideas, and the ideas he has are the usual redistributionist big-government ideas of anybody who spent his time in the Senate (without having accomplished anything during his time there, btw). He supported Arlen Specter and basically supports any of the usual RINO crowd.
He’s a social conservative, but so is Gingrich (anti-abortion, anti-gay marriage), who can defend those positions much more effectively without sounding bitter and priggish.
Santorum far too weak a candidate, almost totally lacking in charisma
And his record of accomplishment is mediocre, at best