Rubio wins outright
Clinton probably wins the general election.
Cruz wins outright
Clinton probably wins the general — but first the nation endures a campaign where one of the two major candidates is an Actual Fascist.
Trump wins outright
This is much like the Cruz-wins-outright scenario. Bad for the country, but at least Clinton probably wins.
Trump runs as a third-party candidate
Cruz or Rubio might win outright, or a combination of deals and shenanigans at the convention gives the nomination to someone besides Trump.
Trump then declares that the Republican Party stabbed him in the back — and he might be right — and that that justifies his running as a third-party candidate.
Clinton probably wins in a landslide, but first the country endures one or possibly two Actual Fascists on the ballot (Cruz and Trump).
This could result in a permanent right-wing nationalist/nativist party like the National Front in France. The Make America Great Again party.
If that happens, look for that party to eclipse the Republican party in just a few cycles. It’s not just about numbers — it’s also about passion. These people are more passionate than old-fashioned Republicans.
Then we’d be back to a two-party system: Democrats and Make America Great Again. You may think our politics are screwed-up right now — but you’d be wrong to think they can’t get worse. Much worse.
(Which only strengthens Trump’s party. Whenever there’s gridlock, look for people to support the party of the easy answers, the party that rails against DC insiders, the party that claims that real leadership can cut the Gordian knot of constitutional governance.)
Perhaps it’s more likely that Trump’s run would be a one-shot deal. There would be no enduring third party.
But I think it’s self-evident that the support for such a party is there, and someone with charisma, money, and megalomania could build it. Someone exactly like Trump.
All he needs is a semi-plausible justification — and not getting the nomination, for whatever reason, is cause enough.