Someone (perhaps plural) will write some kick-ass theses on this Republican primary and the role of the media.
It's been a unique situation since for most of the primary/caucus "season" dating back to last fall, Obama has had high negative numbers and little media focus, so the Republicans have been the only show in town.
My sense is that the modern media doesn't handle multi-player competition well. At the start of the primary season, there were as many as 8 candidates with similar ideologies -- that doesn't work in an era of 30 second sound bites, but a horse race between two people does, and the media began -- with the scantest of evidence -- to promote individual candidates to "contender" status so that they could have a race between them and Romney and thus frame the contest in quick and easy to digest soundbites.
But as each candidate's actual popularity and viability was exposed through increased scrutiny, they fell away, to be replaced by the next in line. Pawlenty, Bachmann, Perry, Cain, Paul, Gingrich, and now Santorum.
Paul is an anomaly in more than one way -- he ran before, so he has familiarity. He also has a sort of hard floor as to how low his support goes due to his previous campaign and dedicated following. Because of this he tends to bob a little in the media, and they never quite write him off like the others.
Santorum I think is the one of the least electable of all of them, but since he's the last man standing Romney has to wrap up the nomination to get rid of him permanently, and the media likes the story of Romney's lack of conservatism and Santorum plays into this beautifully (for the media -- I find him terribly shrill).
At the end of it all, though, it will be Romney v. Obama for president. If the economy keeps pace, Obama will probably win, but only because of Romney's persistent crisis of charisma and many of the born-again bloc voters rejecting him on the grounds he's a Mormon. I don't think it will be any kind of Obama "mandate".