Remember that Romney was unusually blessed in the competitors he faced during the primary. Cain didn't have to turn out to be a buffoon -- but he did. Perry didn't have to prove himself utterly unfit for prime time on a national stage -- but he was. His big competition in the end phases came from Rick Santorum and Newt Gingrich, for Christ's sake. Much the same way that SOMEONE has to win the World Series every year, even if there are no good teams around, someone has to fight out the primary, and whoever does will be regarded in some sense as a person of presidential stature, or at least as a personage capable of putting up a serious dogfight in a political sense. Gingrich and Santorum ended up being held up as genuinely tough competition, because they presented Romney with momentary reasons to become concerned, Romney had to fight them -- a little anyway.
And there's a chicken/egg problem there too, in that maybe Romney is so gifted at political jiu-jitsu that he can make anyone suddenly seem unelectable. But I doubt it. I think the GOP bench is extremely weak, and that Santorum and Cain etc. really are the best competition the GOP can muster right now -- and they're terrible. At no point did the GOP primary resemble Obama/Clinton of 2008 -- or even Bush/McCain of 2000. It was a really weak field, and Romney's nomination (if we remember) was never in serious doubt. Indeed, I can recall a few people writing, "Can you believe this guy's luck?"
Romney is a candidate who cannot present his own record with any clarity, lest it irritate someone in his party's base. Romney's a worthwhile politician who has done genuinely impressive things. But he's snakebit by his divergence from his party's base AND his own status as a billionaire in these 99% times. He has developed something of a reputation in the non-Broderite left as a candidate who lies routinely and recklessly while seldon being called on it, and his periodic pronouncements on foreign policy (going back some years) are ignorant to the point of preposterous.
I think the truth is that Romney is a candidate who basically trades in vanilla and has a few tricks up his sleeve that work under certain conditions -- or work partway under all conditions. He has the usual bag of politician's tricks, in other words, obfuscate when necessary, the "j'accuse" move, and so on. They're not nothing, but they're not really anything special. Until now, sub-special was all he needed to win the primary, and now he's in the big leagues, with triple-A stuff. He's going to be a little bit overmatched.
The main point here is, don't forget how weak his primary competition. He never really got called on a lot of damaging stuff, and he's not battle-tested in the way Obama, Bush, and others were at this stage. He's got a soft underbelly.
Notwithstanding what I've said before, I think there's a lot of truth to this. Romney's just not a good candidate. He got elected governor of a state and he's the bagged the Republican nomination for president. So you can't say he's terrible. And he did bear down and come back from some withering hits in the primary cycle. But at the end of the day, he wasn't facing any serious candidate. The only guy who was remotely a contender was Perry and for whatever reason -- bad luck, painkillers, doofistry -- he just imploded.