I'd say at least half of the whites that voted for him won't this time. Many of the youth vote won't even show up to vote.
The trends reversed in 2010, now those claiming to be Republican out number those claiming to be Democrat.
The key is turnout. If Obama can't jazz his base and get his supporters to show up he won't win. Romney will have little problem energizing his base. Obama does that for him. Obama feels his only chance it to scare enough of his base to give him another 4 years to continue his failed policies. I think that's a tall order. He has a lot of lying to do to accomplish that....but he's willing to give it a go.
First, the notion that "Well, his voters will be disappointed!" Has there ever been an incumbant who didn't go into a re-election campaign with people who felt that he didn't deliver everything they wanted? Yet except for cases where a third party siphons off votes, they always improve their vote totals.
This is the key thing. Getting someone who might be disappointed with Obama. No problem. I'm sure a lot of people who did are. Getting one to say, "Wow, I made a huge mistake, I'd better vote for the guy who came in second to the guy I didn't vote for last time." That's a tall order. A third party often gives these people an out, but voting the oppossite way doesn't.
Meanwhile, Romney doesn't have a lock on McCain's voters. Quite the reverse, some of us voted for McCain in the primaries in 2008 because he wasn't Romney.
If Romney wasn't having a problem energizing the base, he wouldn't have taken "Push Granny off the Cliff" Ryan. He had to throw his base a bone because they generally aren't happy about him.
If hate alone got you over the finish line, you wouldn't have a problem. Hate for Reagan didn't help Mondale, hate for Clinton didn't help Dole, Hate for Bush-43 didn't help Kerry.
I voted for McCain in 2008, but I won't vote for Romney. I just dislike Mormons that much.
You combine the anti-Mormon thing, the don't really trust Romney thing, and the people in the middle who will be just damned scared by the notion of what Ryan is going to do to Granny, and frankly, Romney's got too tall a cliff to climb.
A final note, when you say "Enthusiasm". By that reckoning, there should have been a lot more participation in the GOP Primary process this year. Especially since the Democrats didn't even have a contest, they could have at least sniffed around.
but not really. While 21 million people participated in GOP Primaries in 2008, only 19 million participated in them in 2012.