A chance at what? A chance to see the Congress turn Democratic in 2014, have Romney pass everything they ask because "it's what the people wanted" and then get voted out in 2016 for the most left-wing candidate the Dims can find? If you let the GOP move left, the Dims are only going to move further left, and the American public's going to go left. This is a war. Republicans voting for Romney concedes an enormous amount of ideological territory to the liberals, which will only embolden them and make them try to grab more and more.
The alternative is so much better.