Hard to say what Romney's biggest mistake was; he was running a campaign against a popular, charismatic incumbent with a strong campaign in a time of economic growth. Coming within 4 percent was an amazing result; anyone else would've lost by double-digits.
Economic growth in 2012 was comparable to what it was in 1992...it's just that Obama was a better campaigner and didn't have to worry about fatigue (i.e. Democrats had just be in for one term).
And Obama's approval ratings were mediocre too.
Gingrich or Santorum might've lost by more, but nobody was going to lose by double digits. On the other hand if the recovery had been stronger and unemployment had been sub-6%, then yeah Obama could've repeated his '08 victory even running against Romney.
