William F. B. O'Reilly Portrait of Newsday/amNY columnist Bill O'Reilly (March 28,

William F. B. O'Reilly is a consultant to Republicans.

Momentum is everything in politics, and Donald Trump has it back.

It happened overnight. Just like that.

Trump all but swept the New York primary Tuesday. And almost more important, Texas Sen. Ted Cruz got skunked. Trump won 89 or 90 delegates — ballots are still being counted — Ohio Gov. John Kasich got five or six, and Cruz got zilch. A goose egg.

It was a disastrous night for Cruz — I voted for him — in that he handed what George H.W. Bush called “big mo” right back to Trump after impressively wresting it from the New Yorker in a post-Super Tuesday string of primary and caucus wins.

Trump’s expected to do well in next week’s East Coast version of Super Tuesday, which includes contests in Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island. If he does, he’s going to going to be tough to stop. The May 3 Indiana primary may be Cruz’s last chance to take momentum back before the big kahuna, the June 7 California primary, which comes with 172 delegates.

So what happened in the Empire State, and what hope is left for the #NeverTrump crowd?

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For one thing, Cruz gave up on New York about 10 days before the primary. His team pulled out the state. Maybe that was a brilliant strategic move, but I don’t see it. Cruz could have put at least a handful of points on the board in New York by running targeted TV ads or organizing phone banks, but his campaign chose not to. The result was a big fat zero, a public-relations nightmare.

Another thing happened Tuesday that has largely escaped notice. After all this, after the most talked-about and controversial primary in decades, around 67 percent of New York Republicans stayed home. They simply didn’t vote. Thirty-three percent of Republicans here decided the winner, and it’s easy to understand in retrospect how Trump got 60 percent of them (turnout was higher this year than in 2008 and 2012, but this one really mattered). Trump voters are loyal and motivated; Cruz and Kasich voters, evidently, are not.

For all the talk of this ethereal anti-Trump establishment, there was no trace of one in New York. There were no rallies, no phone banks, no secret websites or handshakes — not even an anti-Trump Facebook page. The only real campaign in New York was Trump’s, and it was effective.

So scattered were non-Trump voters that about 10 percent of them in Westchester County voted for retired neurosurgeon Ben Carson, who dropped out the race more than a month ago. They went to the polls, but didn’t feel they had an actual candidate worth supporting.

Republican TV ad maker John Brabender hit the nail on the head in explaining this phenomenon this week to Real Clear Politics. “There’s been this, ‘We’ve got to stop Trump,’ but not an equal amount of enthusiasm for, ‘Let’s get behind Cruz to do that,’” he said.

That’s exactly right. Trying to stop Trump has never been enough. There has to passion for an alternative candidate.

There’s still a good chance that Trump doesn’t reach the 1,237 delegate threshold to clinch the party nomination before the July 18 GOP convention, but it’s a lot more likely now that he does. But if Trump doesn’t, Cruz should still have the advantage in an open convention. If this goes to a second or third ballot, Cruz may very well prevail.

Right now, all the winds are behind Trump again, and they’re gusting. You can hear it from the chattering class; you can watch Trump’s endorsements steadily build.

But politics is a moody animal. Just like that, momentum can shift back again. It’s what’s keeping a very good Cruz operation working long into these nights. And more than that, it’s the last hope for Republicans like me who fear, to the cores of our souls, that a Trump candidacy could spell the end of the Republican Party as we know it.

William F. B. O’Reilly is a Republican consultant.