Donald Trump never ceases to amaze, but his answer at a CNN town hall about the pledge he had taken to support the Republican Party’s nominee was still jaw-dropping.

Not only did Trump say that the pledge is null and void as far as he’s concerned, he also went further and told CNN’s Anderson Cooper that he doesn’t want the support of Ted Cruz.

Here is a front-runner for a major party’s nomination doing all he can to repel his nearest competitor, who has won 5,732,220 votes so far, or 29 percent of the total (Trump has won 39 percent), and speaks for a significant, and highly engaged, faction of the party. Is there any precedent for such a willfully and pointlessly destructive act in modern American politics?

Every rational calculation says that Trump should seek to preserve the pledge. At this point, he is more likely than anyone else to be the nominee and benefit from the support of his competitors. He should want to use every possible lever of unity at his disposal. And yet, he’s done the opposite.

Who can guess why? Stupid pride? A manliness contest, where he wants Cruz eventually to have to offer his support even after he says he doesn’t want it? A disdain for every political convention, even one that might help him?

Whatever the reason, it is yet another sign that Trump is all about himself. In this sense, he is already what the Republican National Committee feared when it got him to sign the pledge — a third-party candidate. He’s running against the Republican Party from within the Republican Party. He cares nothing about its values or its interests. He favors it exactly to the extent it can be subordinated to him and no further.

Political parties have been riven by clashes of personalities and ideologies before, but it is hard to think of another example of a party so damaged by such a heedless interloper.

It’s been a month since we were told Donald Trump was pivoting to being more presidential and unifying after his victories on Super Tuesday. Since then, he has: declared that he’d consider paying the legal bills of a goon who sucker-punched a black protester; talked of riots at the Republican convention if it doesn’t go his way; threatened and mocked Heidi Cruz; and justified his campaign manager’s manhandling of a female journalist in the most asinine and dishonest ways.

It has become a truism in the coverage of Trump that nothing can hurt him, and with his base that is certainly true. But everyone else has been paying attention, and Trump has made himself toxic with the general public.

Events can always intervene, and Hillary Clinton certainly has her own weaknesses, but every objective indicator is that nominating Trump will mean a divided Republican Party loses in the fall, perhaps badly, maybe even epically.

Probably the most favorable non-Trump scenario is that Ted Cruz beats him on a second ballot at a convention and has enough anti-establishment credibility to take the edge off the inevitable revolt of the Trump forces. But surely Trump would do all he could to destroy Cruz and the GOP in retribution for denying him the nomination.

Trump’s implicit threat is almost certainly lose with me in a simulacrum of a normal process (and lose your integrity and principles along the way), or almost certainly lose without me in an intraparty cataclysm I will make as spectacular as possible. Either way, the GOP is in all likelihood now managing dismal outcomes.

The Trump phenomenon holds important lessons for the party, but there is no escaping the insuperable weakness and failings of Trump himself, namely his egotism, immaturity, irresponsibility and habitual dishonesty.

The RNC thought it had scored a victory so many months ago when Donald Trump signed its pledge. Instead, it was enacting the political equivalent of the fable of the scorpion and the frog.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.