" They will soon be calling me MR BREXIT!" Donald Trump tweeted on Thursday.

I worked for 26 years to make Brexit happen. I wrote the book on why Britain should leave the EU. I appointed the campaign director who won us the referendum.

Mr. Trump, you're no Mr. Brexit.

Here's the difference: The Brexit campaign was upbeat, civil and forward-looking. We weren't afraid of global markets; we wanted to embrace them. Far from opposing a free trade deal with China, we wanted to leave the EU so that we could sign one. We didn't argue that Britain needed to be "made" great again. We think our country is pretty great now, thank you very much. For what it's worth, most of us think the same of your country.

Anger takes a candidate only so far. You can rail against immigration and K-street lobbying and manifold Clinton abominations, and plenty of people will cheer at your rallies. But even those voters who agree individually with every point you're making will often be put off by your negativity. Americans, perhaps more than any other people on Earth, are optimists. They want their candidates to offer something better.

I think Trump's Delphic Tweet was supposed to be about winning elections unexpectedly. We Brexit campaigners certainly did that. On polling day, the bookies' odds implied an 18 percent probability of a vote to leave, 82 percent to remain.

But Donald Trump is not going to win unexpectedly. He is going to lose wretchedly.

For the better part of a year, the Donald spoke mainly about opinion polls. Almost any question, on any subject, would elicit a stream-of-consciousness monologue of how well he was doing in Nebraska or wherever. He is now discovering why other candidates don't do that. If you base your appeal on polling data, you look pretty silly when, as now, the polls turn.

Nor is it just the polls. Hillary is owning the air war, spending $104 million on TV ads against. As far as I can see there is nothing at all from the official Trump campaign. She also seems to have built up a pretty commanding position in the ground war, with campaign offices all over Ohio, Florida and other swing states.

Worst of all, Trump has proven utterly incapable of learning from his mistakes. During the primaries, a few commentators tried to argue — a trifle perfunctorily, it seemed to me — that he would raise his game after the party convention, becoming more scripted and more focused. But if anything, he has become even more foul-mouthed, incoherent and obnoxious. And — guess what? — undecided voters don't much care for it.

Donald Trump's true achievement is that he is the one opponent whom even Hillary Clinton can be certain of beating.

The question for the rest of his party is whether to make his problems theirs. I'm not talking here about Reince Priebus or the RNC, who I think have come in for some very unfair criticism — they had no choice but to follow the party rules. I'm talking, rather, about the other GOP candidates, whose electoral success now depends upon putting as much space as they can between themselves and the man heading their ticket.

Remember the McCain-Obama race? Most people could see early on that Obama was going to win. But instead of concentrating on the congressional elections, the Republicans ran a disastrously personal campaign — "he's a Marxist," "he's a Muslim," "he pals about with terrorists," and even "he's not American." It might conceivably have shifted one or two votes to McCain, but it damaged every other GOP candidate by association.

The one thing we can say with confidence about Trump's campaign is that he will make McCain look like a dancing instructor by comparison. Every down-ticket Republican candidate risks being, so to speak, spattered by the Donald's furious spittle. Are they prepared to give their party an image problem that will last well beyond this electoral cycle? Are they prepared, in particular, to lose, possibly for decades, the fastest-growing demographic group on the electoral roll, namely Hispanic Americans?

Donald Trump came late and malevolently into the Republican Party. It owes him nothing. Almost every observer can now see that his campaign is going to explode. The only question is who is going to be standing nearby when it does.

Dan Hannan is a British Conservative MEP.