Donald Trump doesn’t really care about the Republican Party; he just sees it as a vehicle for his Presidential ambitions. The GOP establishment, for its part, is locked in a shotgun marriage that will end abruptly after he loses. And he will, barring some really compelling assistance from Assange and Putin.
The establishment will try to put as much distance as possible between themselves and the party nominee immediately after the election. They will hope that Trump will simply vanish from the political scene without any further ado. Given Trump’s character, that won’t happen; he will denounce the establishments of both parties, claim that the election was rigged, and call for action in the streets, all of which will be hugely embarrassing to the party leadership and irritating to the GOP’s few remaining moderates.
Will the party split? Paul Ryan and Mitch McConnell will do their best to keep the party united by putting the focus on Congressional resistance to Clinton’s agenda. They will probably succeed, but that is not a sure thing. I would put the odds on a split at 2:1.