You don't. You find someone who can appeal to the ~70% of the country that isn't insane and let the fringes on both sides continue to be insane, ignoring them entirely. That's the way it has worked up to this point. The country didn't just suddenly go insane, it has fragmented for some pretty lousy reasons, which has allowed the fringes to take a larger portion of control than their population warrants. If we had a parliamentary system, we'd be in much better shape because one party wouldn't need to capture 50% of the population by catering to the crazies.