Remember when Michele Bachmann was on top?
I think that when Romney+PACs outspends/out-organizes the other candidates by 5-1 or more, then he wins. Corollary: it is going to be a very expensive primary for Romney and his supporters.
The national polls seem to swing to whoever won the last primary. It would be too easy to make a snarky quip about it, but I really have no idea why Republican opinion should be so volatile.