It wouldn’t screw them over until a legitimate third party is in place. And then maybe. But you have to ask yourself what a third party looks like if it looks like an alternative to both parties. Surely the first third party would just be a split between the parties, it would still take them awhile to win anything at all. But I could see alternatives being a more eco focused party and I honestly think it would screw both major parties. Which is the ideal case. If anything, this screws the dems more because of their voters are more likely to break rank with how the party is going.
As much as I think that the right is going to split after trumps loss, I think they’re also more cohesive than the left is by far. There are many flavors of leftists and while I think that there are also many flavors of right wingers, they have a much easier time banding around their goals because to be honest they don’t do much. They’re a regressive party. Regression is inherently more unifying than progress because we all disagree on how to achieve progress but regress is pretty simple.
Basically my theory is that the right would lose less from a split than the left because half of the left is ready to jump ship at any moment, as is the right, but the right is more cohesive due to shared identity and regression.