This is a good point and a really major question, and one that I have seen debated quite a bit among Republicans.
There has been an active theme on the right this year, arguing that the 2012 election is the GOP’s “last chance” for a while. The argument is that if Romney wins the White House, the party will have some leverage for a while. It wil probably have a few Supreme Court appointments; the modern vetting process is such that people will be chosen young, and for “reliable” views; other changes with some carry-over can be made. Meanwhile, the rural-state skew of the Senate also magnifies GOP influence there.
But if Obama is re-elected, by this argument, the Republicans are in trouble. Their recent positions have weakened them among the following voting blocs: women, Latinos, blacks, gays-lesbians, Asian-Americans, the highly educated, the young. All the growth in the electorate is in these groups. The nightmare vision from this point of view would be California — reliably Republican a generation ago, now the other way.
I would hope that if Romney loses, especially if he loses big, it would be the occasion for systematic re-thought in the party — like what happened after Goldwater (though that led to Nixon) and what happened after the Carter and Mondale defeats in 1980 and 1984, leading to Clinton. My fear is that the hard-liners in the party will say that the problem was that the 2012 GOP was not conservative enough. If only Paul Ryan had been at the top of the ticket.