The following Perry attack ad on Santorum isn’t a major factor in why Perry did so poorly, but it does show what happens when a candidate fully embraces a bizarre talking point and mistakes it for a serious argument (via Andrew):

Perry is trying to bash Santorum as a big-government conservative or statist. That ought to be extremely easy, since Santorum is one. He could have mentioned Santorum’s K Street connections, his support for Medicare Part D (assuming Perry can remember what it’s called), or any of a number of other measures that show Santorum to be a big fan of activist government. Instead, what do we get? Earmarks! Perry chose to focus on one of the most trivial, least offensive parts of Santorum’s record on the flawed assumption that people are deeply offended by this mostly unimportant issue that has next to nothing to do with the deficit. Perry has outdone himself in showing that he is incapable of hitting the broadside of a barn with his political attacks.