People like comebacks. We can identify with the guy or gal who is struggling to redeem themselves (and nobody has ever identified with Mitt Romney before).
Ironically, Romney is almost tailor made to benefit from having lost before. What might be a devastating blow to most political figures — a blight on their resume — actually transforms Romney into a more compelling candidate. Having struggled and stumbled is, for Romney, at least, a feature, not a bug. The same could be said for Hillary Clinton, who only became a compelling candidate in 2008 when she lost her frontrunner status.
People who were hated the first time around — when their lives seemed charmed — can, by facing adversity and overcoming the odds, transform into sympathetic figures — heroes, even — that we actually root for (think Robert Downey, Jr.). Nobody has ever rooted for that scrappy Romney kid to overcome the odds. Until, maybe, now.