What a nifty coincidence, if that’s what it is. Just as Mayor de Blasio has put New York City into a downward dive on the road to the 1970s, New Yorkers are about to get a 50th anniversary edition of William F. Buckley Jr.’s book about running for mayor.

Buckley’s 1965 mayoral campaign failed. He won but 13 percent of the vote in the election that handed up John Vliet Lindsay as the 103rd mayor. But the campaign revitalized the conservative movement in New York — and America.

That’s a fact to savor at a juncture when, for the first time in a generation, the city is suffering under an all-Democratic city government. As things go downhill from the Giuliani-Bloomberg years, there is no one to blame but the party of Bill de Blasio and Melissa Mark-Viverito.

This is more dire than 1965. Back then, the Republicans still held, via Nelson Rockefeller, the governorship. Robert F. Wagner Jr.’s mayoralty was ending; the Democrats nominated Abe Beame.

The GOP chose as liberal a Republican as there was.

Lindsay was so liberal that when asked to explain his vote in Congress against legislation to combat pornography and Communism, he famously replied that they were major industries in his district. Lindsay was way to the left of the national Republican Party and Buckley way to the right of the state’s GOP.

Buckley ran on the ticket of the newly formed Conservative Party. “Why didn’t you run in the Republican Primary,” he later asked himself in a mock-interview that is included in the memoir, which is called “The Unmaking of a Mayor.”

“Why didn’t Martin Luther King run for Governor of Alabama?” is the quip with which he answered his own question. The memoir is full of the jaunty arguments and ripostes that made the founding editor of National Review such a celebrated figure.

To read Buckley’s memoir 50 years later is to be reminded that he lit up the sky over issues that are back with a vengeance. His chapter on “race, religion, and politics” is only one of the sections of the book that could have been written yesterday.

It includes excerpts from the columnist’s testimony to the City Council, where he warned against “pressure to encumber the police” and pleaded for a minority community that was the principal victim of crime committed by minorities. Sound familiar?

Buckley also focused on corruption, taxes, federalism and the clash between interest-group politics and universal principles. He ended his book with a chart of the rise in welfare spending between 1935 and 1964, when it had soared 4,500 percent to $630 million (chicken-feed compared to today’s $14 billion in social services outlays).

Although Buckley ran as a Conservative, he expressed regret at what he called the “prospective decline of the GOP.” The alternative, he warned, was a “congeries of third parties, adamantly doctrinaire, inadequately led, insufficiently thoughtful, improvidently angry, self-defeatingly sectarian.”

That’s something Buckley never was.

On the contrary, Buckley maintained a broad range of friendships. The first time I had dinner at his Upper East Side home, I walked in to discover the host, America’s most famous conservative, huddled on the couch — almost “politically necking,” I later wrote — with the country’s then most-famous liberal, Mario Cuomo.

What a manual this book is for conducting a principled, good-humored, inspiring conservative campaign in a liberal city. No doubt there will be those who will gloat over the fact that Buckley got shellacked in the 1965 election. But that was a battle, and the war is long.

By running the race he did, Buckley inspired a whole generation of conservatives and vaulted himself into the vanguard of the conservative movement well into the 21st century. It is a brilliant move by Encounter Books to bring out this volume anew.

It will give a lift to New Yorkers who are starting to think about what comes next. And, as Buckley’s friend Neal B. Freeman is quoted in an Encounter press release as saying: “It’s been fifty years now since Bill Buckley demanded a recount. Perhaps we owe him one.”

This column first appeared in the New York Post.