Donald Trump, left, and Anderson Cooper in Milwaukee on March 29. (Darren Hauck/Getty Images)

MANY TIMES since the New York real estate mogul commenced his presidential campaign, wishful analysts have declared “Peak Trump.”

Remember when The Donald sneered at Sen. John McCain’s (R-Ariz.) war record? Or crudely mocked a New York Times reporter’s disability and then denied having done so? Or made bizarre insinuations about Fox News anchor Megyn Kelly’s hormonal balance? Or shrugged at a Klansman’s support? Each transgression prompted pundits to predict that, at last, the GOP front-runner had gone too far and that his improbably high support within the GOP primary electorate would collapse. On each occasion, he kept winning, buoyed by a loyal, large minority of Republicans who are “supportive of Trump for reasons that defy some of the facts even,” as Doug Sachtleben, communications director of the conservative Club for Growth, which has been spending heavily on indifferently effective anti-Trump advertising, told the Hill newspaper.

It would therefore be foolish to speculate about the political impact of Donald Trump’s performance over the past two weeks — except to say that it represents a critical and, we would argue, irrefutable mass of evidence that he is unfit for the presidency. If this doesn’t blunt his drive for the Republican nomination, it’s hard to say what would.

What’s new is the sheer accumulation of statements showing that Mr. Trump has not thought seriously about major issues, including some that are anything but obscure, such as abortion — about which he first opined to MSNBC’s Chris Matthews that women who terminate pregnancies should be punished, then retracted his view.

The streak of strange utterances actually began in his disjointed March 21 interview with us, which produced his assertion that the United States is “a poor country” whose security alliances may no longer be affordable. He suggested, in an interview with Anderson Cooper of CNN, that the United States should stand aside as Japan and South Korea develop nuclear arsenals to protect themselves. As for Saudi Arabia, Mr. Trump first said “absolutely” it should get nukes then reversed himself in response to Mr. Cooper’s incredulous follow-up. And though it hardly seemed possible, the candidate broke new ground on incivility as well, defending his campaign manager, Corey Lewandowski, from a charge of assaulting a reporter — despite compelling video evidence that the incident did occur and that Mr. Lewandowski was wrong to call his accuser “delusional.”

Republican presidential front-runner Donald Trump visited the editorial board of The Washington Post on Mar. 21. Here is audio of the full, unedited interview. (Gillian Brockell/The Washington Post)

Mr. Trump now trails in polls for Tuesday’s Wisconsin primary; reflective of that state GOP electorate’s special characteristics, no doubt, but also a wider level of opposition to the front-runner that has him recording an unprecedented 67 percent unfavorable rating in the latest Post/ABC News poll. If he loses Wisconsin, it may be impossible for him to claim a delegate majority before the national convention; there may even be negative repercussions for him in subsequent primaries, such as New York and California, where he currently leads.

The upshot, for the Republican Party’s leadership (such as it is): Denying Mr. Trump the GOP nomination, which has always been imperative, is now not only more urgent, but also, perhaps, more feasible. They should redouble their efforts accordingly.