I will offer my thoughts, and I'm curious what others think about how the Republican field will change as the lesser candidates drop out of the race.
PREFACE: I know many of you on this site 1) don't think anyone other than Ron Paul should even be discussed because they're no better than Obama; or 2) think we're all doomed and politics is nothing but window dressing for the control of our world by power elites. This topic is not for you.
CW has been that Romney's nomination is inevitable, and Intrade puts his odds of victory at 53.5% - I disagree completely, this is a sucker bet. If I were advising him, I'd tell him he has very little chance of winning as things stand now. He's the only "moderate" "establishment" Republican polling any real numbers, with fellow Mormon Huntsman being the only other in the race. I don't see any bloc of voters supporting any other candidate that he will pick up when they leave the race.
Cain appears to be done, so I don't think it's worth analyzing where he could draw future support. If he leaves the race early, I think most of his voters go to Gingrich. They're people that decided they didn't want the front-running Romney and are economic conservatives first. Gingrich is their most likely destination, further strengthening his candidacy.
Perry also appears to be done, but could resurrect (pun intended) his candidacy. You don't get elected Gov. of Texas with zero political skills, though don't confuse that with IQ. Texans don't seem enamored with especially smart politicians. If Perry hangs in there, he would be the natural home for Bachmann and Santorum voters, and there could be an establishment revolt against Gingrich as unpopular with the country club set, and Romney as unable to win the nomination (ironically enough). That could move lots of establishment types and money to Perry, who had a strong fundraising showing early.
Gingrich is growing his support organically, but would probably inherit most of Cain's support if and when he leaves. Gingrich is in the best position right now - he isn't really concerned with Romney, but is moving to the center to openly court his voters because he's even less concerned about any candidates on the right. If I had to guess, he's probably most concerned with Perry but figures he'll take his chances with him, because one on one he'll absolutely annihilate him in a debate. Besides, Perry's polling in the single digits. At 28.5% odds of getting the nomination, Gingrich is a great value bet because I actually think he will soak up most of the free voters and beat Romney...
In a 2-man race between Gingrich and Romney, which is looking ever more likely, I think Romney is maxxed out at 40%. I just don't see where the other votes come from absent Gingrich making a huge mistake. Romney is polling best in NH at 34%, and I just don't see where he gets much more support than that as the field thins out. The candidates that will be leaving are right of Gingrich, not left. Unless the Republican mood changes, primary voters without a real Tea Party candidate will see Gingrich as the next best thing.
Let's see... who am I forgetting... Oh yeah, that Paul fellow. He's polling healthy double digits everywhere except South Carolina, and my guess is he'll be in the race longer than Cain, Bachmann, or any of the other also-rans. It is not inconceivable that he could grow his support north of 20% in certain states. A surprise showing in New Hampshire could get him some much needed press. It is difficult for the press to attack Paul, because he doesn't leave any of the easy openings they're used to exploiting - flip-flopping, scandal, hypocrisy. It is one thing to dismiss him as a "fringe" candidate when he's polling 5%, but as Cain can tell you, polling in the top 3 gets the media's attention and they actually look at your policy prescriptions. The media may think Paul's are nutty, but it's harder to distort his message than it is to dismiss it. Once they actually have to discuss it, it could be a game-changer. Intrade puts his odds at 5.2% currently... by far the best value bet in the race.