Their egos blind them...
The Trump movement is a no win situation for the GOPe. Should a Trump presidency be a success, the GOP as we know it will be permanently changed. Those who resisted the move to populism and nationalism will be forever on the outside looking in.
If Trump is a colossal failure, the GOP as a whole is toast. Those who opposed him, still go down with the ship.
Many will work to undermine a Trump administration for precisely as you stated, ego. Never mind the consequences.