What's your thinking here? R and D are (presumably) zero-sum. Either with each other or with some 3rd/nth party. But there's a steady state in this system where R=D=0. Or are you modeling something other than actual seats held I guess?

I mean right now both R and D are extremely unpopular, probably in large part because of lack of bipartisanship (at least doesn't help). But R still hold a crap load of power.