If this happens, this will be a 1964 style landslide, with Republicans losing the Senate and the House.
It would be a 1964 style landslide on the presidential level, but why would it entail the Republicans losing congress? I imagine both Trump voters and Pubs voting for Kristol's neocon will both vote for Pubs downballot.
It would mean the party tearing itself apart, with Republicans staying home en masse because the outcome of the election would be predetermined. Republicans can't possibly be that stupid, can they?
1) The party is already shredded. Look at the list of those skipping the convention.
2) With the nomination of Trump, Republicans have demonstrated that there is no lower bound to their collective stupidity.