If you can get beyond the 'gee, anyone would want this honor and chance to serve my country', I think it's worth looking at what the possible VP candidates must be thinking.
Here are some of questions for them to answer:
1. Will we win?
Question 1 is tough, but seriously, it's getting less and less likely every day. Even with all the garbage that the Obama campaign (and others, like us!) have beein digging up and dumping on Romney, one MUST believe that the Obama campaign hasn't fired its heavy artillery yet. The economy isn't going to get markedly better in 3 months, but it isn't going to get markedly worse, either, so that's not a real player.
The worst part for a potential VP candidate is contemplating joining a campaign that is just plain HORRIBLE! These people can't get their foot out of their collective mouths, can't get their strategy straight, and persist in insisting on letting Obama and his campaign define not only their candidate, but also direct the entire tone and content of the election. So who wants to joing that traveling circus?
2. What happens to me if we don't?
Question 2 isn't a happy one to contemplate. Consider: Walter Mondale, Geraldine Ferraro, Lloyd Bentson (had to look that one up!), Dan Quayle, Jack Kemp, Joe Lieberman, John Edwards, Sarah Palin....... Not one of those august collection did anything after their losing VP effort.
Simply put, the losing VP candidate will have committed political hari-kari. Maybe, just maybe, Paul Ryan or Bobby Jindal are young enough to recover some national mo-jo in 20 years, but is that what they are really aiming for?
3. Can I make the difference?
Historically, probably not. Consider: George H. W. Bush, Dan Quayle, Al Gore, Dick Cheney, Joe Biden.... how many of those guys really turned the elections(s)? That would be a subject for an interesting debate, but nobody on that list jumps out of the screen, does it?
4. What happens if we WIN?
Same list as in #3 above. Other than HW, nobody went anywhere, even after winning. (Agreed, not a very large sample - feel free to push this analysis further into the past, but I don't think results more than 40 years ago really reflect the current political landscape.)
So based on those 4 questions, here is my handicapping on the current 'conventional wisdom' Romney short list:
Rob Portman - Age 55 or so, won't get any higher if he doesn't take the VP slot (other than perhaps a cabinet slot), can keep his Senate seat if they don't win. Not much to lose. Might help win Ohio, but not a slam-dunk.... Romney can't win without Ohio, but can't win with only Ohio (out of the swing states). Won't help much outside Ohio, but other than the stain of being Bush's budget director, won't hurt much, either. Wouldn't be a dynamic candidate to succeed Romney if they did win.
Tim Pawlenty - Age 51, unlikely to win anything in the future on his own, so nothing to lose. Won't help Romney win any state, but - being boring like Portman - probably won't hurt either. Wouldn't be a dynamic candidate to succeed Romney if they did win.
Paul Ryan - Age 42 - Probably wants to be President himself. Won't help Romney carry any state. Touted as the best person to be able to explain the 'Ryan' budget, but seriously... how can anyone make that case? Would take 20 years to recover from losing - probably willh have to give up House seat to run and lose. Could use a win as a springboard to succeed Romney, but hard to believe he wouldn't get bored to death as VP.
Bobby Jindal - Age 41 - Would love to be president. Won't help Romney carry any state, could hurt the campaign. Has the 20 years necessary to clean up the stain of losing. Hard to envision as a presidential candidate.
I'm not going to handicap Christie, Rubio, Ayotte or McConnell, or any of the others... feel free to give your own input.
So out of this list of 4, my take is that Portman or Pawlenty have nothing to lose by accepting, but Ryan and Jindal have everything to lose.