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.
Hopefully President Clinton's first action is banning this despicable party in that case. Country before party after all.
Conceding 1 election vs conceding the next 20 years due to how badly Trump will damage our brand.
What brand? The Republican brand is already irreparably damaged and incapable of winning the presidential election ever again.
Trump is the last hope.