Romney would be an epic disaster. His obstinate, egotistic refusal to disavow Romneycare means that, were he elected, liberals/democrats would immediately latch onto his election as "proof" that the "consensus" of American voters is that the country wants Obamacare, and he would be powerless to prevent himself from being so used (even if he doesn't affirmatively go along with them).
That leaves no alternative.