The problem with Santorum this time is that he never got started and it's too late now. Santorum is a good guy, but he is overwhelmed by the current people, and will probably withdraw when either Trump or Cruz wins Iowa
In 2016, conventional wisdom seems to have gone out the window 
The field in 2012 was very weak, and dissatisfied voters coalesced around him as the "not Romney." This time, the field is much stronger, so he's having a hard time breaking away. Also, only a handful of candidates hold a huge amount of the support both in Iowa and nationally (Trump+Cruz+Rubio+Carson=79.1% in the Iowa RCP average) and this makes it hard to break away from the pack. There are several candidates (Huckabee, Cruz, Paul to an extent) splitting his base of evangelical voters and born-again Christians that make up over 50% of caucusgoers. Finally, at this time in 2012 Santorum had 18% support in Iowa and still only squeaked out a victory; now he has 0.5%. A Santorum surprise, whether a win or just placing in the top five, is unlikely.
