I don’t think Obama’s defeat is inevitable at this point. It’s a long way to next November, and he is devious, well-funded, and willing to do nearly anything to get what he wants. Right now, it sures looks like an empty suit could beat Obama handily, but we shouldn’t underestimate him.
Also, the main reason why I say it would be a disaster for the party is that the party needs the conservative vote more than the conservatives need the party. No Repub presidential candidate will ever get elected if conservatives stay home in large numbers. So the party needs the conservative vote to win, and if they nominate Romney they are doing the worst possible thing they could do in terms of boosting conservative turnout.
Conservatives, on the other hand, don’t really need the Republican party, since we hardly get anything from them that would be too dear to give up. They haven’t let us have a conservative presidential candidate in 30 years, and even when we work our butts off to get the party majorities, they squander them and spit in our faces. We mainly get lip service from the party, and we can get that from any number of places if we march our votes elsewhere.