First of all, Romney is a weak campaigner, as evidenced by his inability these past several months to "close the deal" with Republican voters. Rhetorically, Obama can run rings around Mitt.
Secondly, Obama (and I mean, campaign-mode Obama) can still inspire, whereas Mitt cannot, even among his most ardent supporters.
Thirdly, there is a populist movement — which crosses party — lines relating to income inequality. The Romney camp is already being beaten up about this, and their defense — "hey, don't punish success" — isn't going to fly. Why not? It's easily rebuttable. Being successful in a capitalist system is fine, but being successful at the expense of the middle and lower classes? Not so much.
Fourthly, Romney can easily be painted as the class of person that was responsible for getting us into this financial mess in the first place.
Fifthly, Romney cannot go after Obama with any credibility regarding "Obamacare", since much of it is patterned after Romney's own plan.
Sixthly, although parties generally unify around their final candidate, the conservative base may not flock to Romney as much as he would hope. They might sit out this election.
Seventhly, Romney can't connect to immigrants and minorities (Newt has better outreach because of his immigration policy).
Eighthly, Romney, unlike Newt, will not try to win votes by tapping into the racial divide.
Unless I'm wrong about this… in which case…
First of all, Newt's strongest appeal is with the conservative base. His policies and his attitude may fire up that base, but he will have a much harder time reaching moderates, independents, and swing voters. And (they say) elections are won or lost on the moderate, independent, and swing voters.
Secondly, Newt cannot reasonably call himself a leader, nor a Washington outsider. As Speaker, he was ousted from the leadership. And he has spent the last 14 years as a K Street lobbyist.
Thirdly, unlike with Romney, Obama can draw a deeper contrast between himself and Newt (and vice versa). There is less chance that one will draw from the other's potential votes than in an Obama-Romney contest.
Fourthly, although he is a good debater, Newt has a propensity to shoot himself in the foot with his grandiosity.
Fifthly, Newt doesn't have the money or organization behind him. While this might be seen as a virtue, it makes him more likely to make strategic errors.
Sixthly, there is a LOT of usable footage of Newt saying things that he totally disavows now (like how he was once in favor of an individual mandate).
Seventhly, Newt has a +34 unfavorable rating with the American public. The latest PPP poll shows that 60% of Americans have an unfavorable opinion of Gingrich, compared to only 26% who like him. Ouch. Mitt Romney's, meanwhile, is only +18 unfavorable.
And there it is. My analysis. What's THE answer?
Beats the hell out of me.