I know exactly when I grasped that Donald Trump had a chance of winning Tuesday’s election: July 26, 2023, when I watched a line of people form outside Midtown’s Roosevelt Hotel.

The crowd of migrants — mostly young men and mostly Hispanic and African, with some Europeans as well — grew for six days in the sweltering heat while New York City’s government looked on helplessly.

Those enduring images demolished President Biden’s, and later Kamala Harris’, best chance to keep the White House.

The scene pulled the pin on a notion then prevalent among many voters: that progressive Democrats may be nuts, but those billed as moderate, like Biden and like New York’s governor and mayor, are capable of setting and executing responsible policy.

How did these images come to dominate national news?

First, Biden wanted to demonstrate how mean and heartless his predecessor was.

His very first day in office, Biden suspended a key Donald Trump policy that had kept the southern border from being overwhelmed by “asylum seekers,” many with no credible claim to asylum — the rule that had them “remain in Mexico” until their cases were heard.

You may revile Trump, but two things are true: The United States cannot accommodate an unlimited number of arrivals who step across the border in a disorderly fashion, and “remain in Mexico” was keeping that from happening.

Biden’s reversal sent a clear signal: America had reopened — to everyone.

As even The Washington Post acknowledged, “Illegal border crossings soared to record levels under President Biden, averaging 2 million per year from 2021 to 2023,” compared to about 800,000 in 2019.

Biden “paroled” millions of border crossers into the country, 1.3 million in 2023, easily five times Trump’s level. Millions more have evaded any detection at all by border officers and have gone uncounted.

Then, Mayor Adams put swagger before substance.

In April 2022, Texas Gov. Greg Abbott, faced with the logistical nightmare of dealing with hundreds of thousands of unvetted people with no help from the feds, began regularly busing migrants to Washington, DC.

That had nothing to do with New York — but Adams, apropos of nothing other than wanting free ink, called Abbott a “coward” in response.

“Our country is home of the free, land of the brave,” Adams mused. “We do not . . . send people away who are looking for help.”

It was an open invitation — so Abbott began sending buses to Gotham.

Adams could have quickly realized that finding shelter for tens of thousands of asylum seekers was untenable. He could have petitioned the courts and the state to limit right-to-shelter rules that require the city to house all comers.

Instead, Adams began opening hotel after hotel, tent city after tent city — 212 sites now, housing 62,000 people.

Adams didn’t even try to seek to limit the right to shelter until May 2023, and then only to institute 30- and 60-day time caps.

Finally, Gov. Hochul did nothing, other than throw cash at the problem.

Hochul could have stepped in early and curtailed the right to shelter, which is a state matter.

But she let Adams deal with the mess, forking over at least $2.6 billion in state taxpayer funds to do so.

In July 2023, it all culminated in scenes of chaos at the Roosevelt Hotel, which City Hall had opened as a shelter.

That Adams allowed migrants to wait outside this high-profile Midtown property just a block from Grand Central Terminal for days — without even seeming to notice it — showed sheer incompetence.

The country’s voters were watching, though.

What did they see?

Biden, Hochul and Adams weren’t elected as “defund ICE” progressives, but as moderates (and Harris has positioned herself in this race as a moderate, as well).

Nor were the federal, state and local governments experiencing any lack of financial or operational resources.

The problem is simply that the United States does not have the ability to feed and house the entire world.

If New York, the richest city in the country, couldn’t deal with this influx of newcomers, how could other cities and even small towns manage it?

National Democrats weren’t remotely interested.

The Biden-Harris administration didn’t even begin to attempt to reduce the migrant flow until this year — and only because top national Democrats finally realized, two years too late, that the border crisis could lose them the White House.

This week, with 61% of voters viewing immigration as a critical issue, we’ll find out if it did.

If so, this election was decided two summers ago, in the middle of Manhattan.

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.