Romney is in command of the board going into Super Tuesday. It's unlikely that any of the candidates will make a move before then, but once the results are in somebody is likely to fall off the map.
Romney will fight to the bitter end unless something insane happens. One does not simply back out without fighting after leading for so long.
Santorum will need to win two states with Washington, Ohio, Oklahoma, and Tennessee being the most likely victories. If he comes out with one (or none) his campaign will be seriously derailed.
Gingrich has to win Georgia and place well in some of the other states to keep his thin chances from breaking.
Paul must put full effort on Virginia to stay in the GOP race (though a run as an independent isn't out of the question).
Who will drop out first after Super Tuesday?