My opinion: He's definitely running. He definitely WON'T be the nominee.
I think Santorum is the favorite at this point, although there's not much point in handicapping the race so early. He was the strongest challenger last time, and the GOP usually goes with that. And the conservatives will demand one of their own.
I think Gingrich was stronger. Regardless, the "next in line" pattern that Republicans like to follow doesn't always mean whoever finished 2nd in the previous election's primaries. It can be the previous VP nominee (Ryan) or maybe the candidate people wanted last time but declined to jump in (Christie, Rubio, J. Bush).
2016 will be unique though and the GOP primary voters may break with habit. '08 we could blame on the economic collapse and Bush's unpopularity, but 2012 was a gut-check. We might find ourselves in a situation similar to the 1964 primary battle.
