Donald Trump is being badgered in Wisconsin today, on the heels of a defeat, at North Dakota, where most delegate chosen this weekend were supporters of Senator Cruz, albeit soft unbound supporters. Mr. Trump will point ahead to New York, where he is likely to win, as any candidate should be able to do in his home state.

Mr. Trump may try to shrug it off by saying, “You win some, you lose some.” Actually, winning candidates don’t lose this late in primary season. Just as the rich get richer, winners get more victorious. Mr. Trump isn’t. In 2012, Governor Romney — “the Choker” by Trump’s description — suffered his last loss in March and, by early April, had accumulated more than 900 bound delegates. Mr. Trump’s still losing and has but 736 delegates.

More than anything else, Donald Trump has lost the Big Mo. At a time when a frontrunner should be increasing his momentum, he is decelerating. When a leader should be forcing the competition to capitulate, his opponents are fighting back harder than ever. Even the hopeless governor of Ohio, John Kasich, is forging onward. When Mr. Trump should be projecting inevitability, he is suddenly evincing vulnerability.

Tomorrow morning, The Donald will still be stalled at about 737. He’ll still need 500 more bound delegates, but the pool available in future contests will have dwindled to about 715. He will need to win 70% of that pool, a virtual impossibility for a stalled candidate. The author of “The Art of the Deal” can’t close the deal. Imagine, the candidate who can’t close is the one who has been running around the country talking about the amazing deals he will strike, the ones where he’ll outsmart all the foreigners who’ve been taking advantage of America.

So why is the New Yorker backsliding? Foremost, his head-to-head polling against the likely Democrat nominee, Hillary Clinton, has deteriorated. From early September to early March, he bounced between 3 points and 6 points to the negative of Mrs. Clinton. Supporters could entertain the notion that these numbers would improve as Candidate Trump “matured.” Yet over the last month, his negative gap has widened to 10 points.

Republicans want to win. They are backing away from a candidate who’s looking more like a sure loser. Mr. Trump has, at least to some, become a cancer. Over the September to early March period, his net unfavorable rating with all Americans swung between the negative teens and twenties. Since early March he has gapped down into the negative thirties. Two out of three Americans view him unfavorably. The Donald is endangering every down-ticket GOP candidate nationwide.

So why these trends? He has now insulted just about every possible group in the nation. At first, it was just Mexicans. (Nativists could chortle.) Then it was women. (Macho men could snicker.) Next were his fellow candidates. (Spared candidates could relish the takedowns.) When debate audiences booed, they were the hated “establishment” or “the lobbyists who buy and sell the guys to my right and left.” (The self funder could preen.)

Second, Mr. Trump has disclosed ignorance of important issues. In December’s debate, he failed to comprehend the nuclear triad. “Biggest problem, to me, in the world,” he told the Times last week, “is nuclear, and proliferation.” Then, when Anderson Cooper pressed him, “you said [to the Times] you might support Japan and South Korea developing nuclear weapons of their own. Isn’t that completely contradictory,” he replied “No, not at all.” He explained that they have to pay us for our nuclear umbrella or they have to develop their own. The Wall Street Journal explained two days later that they do pay us.

Then there was his assertion, and immediate reversal, that women should be “punished” for getting abortions. And, a while ago, he flip-flopped on H1B visas within just 24 hours. The list goes on. If you press Mr. Trump’s supporters about his policy ignorance, they say he’ll learn, he’s a smart guy.

If you argue that the job, and the campaigning to obtain it, is so intense that it leaves little time for on-the-job learning, they rejoin that he’ll surround himself with good advisers. Well, Mr. Trump is on the record saying he is his own best adviser. But let’s suppose he relents on that score. After all, he has announced a small team of [completely unknown] foreign policy advisers. The problem is that you have to know enough about an issue to know good advice from bad. Trump doesn’t.

The Trump train is decelerating for good reason.