Via the always interesting Richelieu:

Here is a farfetched but not impossible scenario you can use to wow your friends at cocktail parties: Rudy continues to drop in the polls. His campaign is indeed broke. The media continue to hammer him on ethics. He finishes a dismal fifth or even sixth in Iowa. Polls show him third or worse in New Hampshire. With his campaign broke and without money to run TV ads, Rudy faces a stark choice. He can either run and badly lose the next four contests - NH, MI, SC, NV - and then collapse in Florida. Or he can make huge brazen move to capture the entire race.

On the Saturday after the Iowa caucus, Rudy Giuliani drops out of the race and strongly endorses John McCain. McCain surges and wins New Hampshire. A national McCain surge accelerates. McCain campaigns with Rudy at his side, who is obviously the frontrunner now for vice president on a McCain/Giuliani security-and-competence ticket. In 24 hours Rudy goes from doomed to Superman. With McCain likely to only serve one term, Vice President Giuliani enters 2012 as the GOP frontrunner. You can argue a similar scenario for Romney, but the McCain scenario is more plausible. The two are friends, and McCain's age makes him more likely to serve just one term.