Do you realize that even a small split, where say just 1 out of 10 Republicans walk, that it is the Democrats and no one else that will benefit? If just a splinter of a splinter of Republicans walk wholesale, the Democrats will win a huge majority and the presidency.
It depends.... I get the sense that there are really three main groupings of political beliefs these days: A moral conservative grouping, a left-wing/green grouping, and a centrist, moderate/libertarian grouping. The moral conservatives are about 35-40%; the left-wing/greens are about 15-20%, and the center is about 40-50%.
The Democrats stay close by keeping the left-wing greens, black voters who are moral conservatives, and centrists who are more scared of moral conservatives than the left.
However, if there was a split in the GOP, the black moral conservatives could leave the Dems (especially if the conservatives took some other name than "Repblican") and the moderate Dems could well join the centrist party. We'd have, at least for a little while, a party system that represented most Americans without any faction able to pin down anything more than a bare majority.