Was Romney's defeat one of the GOP's most disappointing?

[Pericles]

Yes. Romney was leading in the national popular vote for a while before the election. We saw how NV, CO, IA, WI, OH, NH, VA and FL were all within reach for Romney.

The national polls underestimated Obama though. Obama won NV by 7 points, CO by 5 points, IA by 6 points, WI by 7 points, OH by 3 points, NH by 6 points, VA by 4 points, and FL by 1 point. Remember Obama won the popular vote by 4 points, so he had a notable advantage in the Electoral College.

[Pericles]

Yes. Romney was leading in the national popular vote for a while before the election. We saw how NV, CO, IA, WI, OH, NH, VA and FL were all within reach for Romney.

The national polls underestimated Obama though. Obama won NV by 7 points, CO by 5 points, IA by 6 points, WI by 7 points, OH by 3 points, NH by 6 points, VA by 4 points, and FL by 1 point. Remember Obama won the popular vote by 4 points, so he had a notable advantage in the Electoral College.

I do believe Romney would've won most if not all of these states if not for some mistakes near the end of the campaign.

You can believe that, but there isn't any evidence to convincingly argue that Romney could have made up those margins with a change in October. It just doesn't seem plausible.