It wouldn't end well for the GOP. Even if they just concede the presidential race, such a chaos, culminating with trouncing the presumptive nominee, would hurt them in downballot races.
Sometimes you have to put the country before party. I'm ok with conceding the Presidential election in this circumstance.
I would respect a Republican who took this stance. I have little respect for those who trash Trump, but say they're voting for him because Hillary is so bad. That's ridiculous on its face.
It's ridiculous to believe there's a lesser of two evils?