Donald Trump may have eased some Republican fears Tuesday night when he declared his intention to stay inside the party. But if their angst has been temporarily eased at the prospect of what he would do if he loses, they still face a far more troubling, and increasingly plausible, question.

What happens to the party if he wins?

With Trump as its standard-bearer, the GOP would suddenly be asked to rally around a candidate who has been called by his once and former primary foes “a cancer on conservatism,” “unhinged,” “a drunk driver … helping the enemy.” A prominent conservative national security expert, Max Boot, has flatly labeled him “a fascist.” And the rhetoric is even stronger in private conversations I’ve had recently with Republicans of moderate and conservative stripes.

This is not the usual rhetoric of intraparty battles, the kind of thing that gets resolved in handshakes under the convention banners. These are stake-in-the-ground positions, strongly suggesting that a Trump nomination would create a fissure within the party as deep and indivisible as any in American political history, driven both by ideology and by questions of personal character.

Indeed, it would be a fissure so deep that, if the operatives I talked with are right, Trump running as a Republican could well face a third-party run—from the Republicans themselves.

That threat, in turn, would leave Republican candidates, contributors and foot soldiers with painful choices. A look at the political landscape, the election rules and the history of intraparty insurgencies suggests that it could turn 2016, a year that offered Republicans a reasonable chance to win the White House and with it total control of the national political apparatus, into a disaster. ...