My only worry with Mitty is can Dems put him in via crossover voting, knowing that it will incite a 3rd party run and a likely Clintonian plurality win for Obama.
Besides, most liberals really believe that Mitt is the most formidable candidate because he's the only one who sounds reasonable to them, with all his big government solutions and oily non-answers to controversial issues.
They would vomit if they ever had to actually vote for Mitt, but they'd vomit less for him than any other Republican in the race. And they think that's why we're stupid for not wanting him!