Fivethrityeight.com has an interesting analysis on this. It shows that the loss/gain is linearly correlated with POTUS popularity with the break even point at 55%

But the demographics are awful for the GOP Go look at the Roper Poll on age based election demographics. The years after Reagan are 100% consistent with the statistic that how you voted in your first national election is 75% predictive of how you vote for the rest of your life.

And this is where the GOP is in deep kimchee. Obama got 65% of the 18-29 yo vote. GOP got 33%. This is catastrophic for them based onthe Roper data