As Cynthia Nixon gains ground on Gov. Andrew Cuomo, my warning to New York Democrats to be careful what they wish for applies in spades to Mayor Bill de Blasio. Nixon’s growing support means the day is fast approaching when de Blasio must make a hard choice.

Will he publicly endorse Nixon in the primary against his archenemy, the governor?

Either way, there are huge risks, and being wrong will be painful.

This is no ordinary moment in New York politics. The feud between Cuomo and de Blasio is not unusual for mayors and governors of the same party, but for either one to endorse an opponent is very unusual.

Insiders can cite only two examples in the last half-century, and both involve the governor’s late father, Mario Cuomo. In 1977, Gov. Hugh Carey, a Democrat, encouraged Cuomo, then his secretary of state, to challenge incumbent Dem Mayor Abe Beame during the fiscal crisis.

Cuomo survived a crowded field to get into a primary runoff with Ed Koch and, despite Carey publicly repeating his endorsement two days before the vote, Koch won the nomination. He defeated Cuomo again in the general election when Cuomo ran on the Liberal line, with GOP nominee Roy Goodman finishing a distant third.

Fast-forward to 1994, when rookie GOP Mayor Rudy Giuliani shunned his party’s nominee, George Pataki, to endorse Mario Cuomo for a fourth term as governor in the general election.

Giuliani’s decision grew out of a GOP feud centered on his fractured relationship with Sen. Al D’Amato, with the mayor charging that “ethics will be trashed” under Pataki. Those words came back to haunt him for years as Pataki, after his victory, often made life difficult for Rudy.

While politics and the state have changed, the fact that Mario Cuomo lost both those races should give de Blasio pause about endorsing Nixon, especially when Andrew Cuomo is the clear front-runner. A Tuesday Siena poll shows him leading among registered Democrats 58-27 percent, down 16 points from the 66-19 percent lead he held before Nixon formally entered the race.

Yet because Nixon is the mayor’s stalking horse, some observers think it’s certain he will endorse her. They argue that if he doesn’t, his progressive bona fides would be severely undercut.

And that could end his political career because he is term-limited as mayor.

But there is another dimension to the game now that Nixon has won the endorsement of the Working Families Party. Even if she loses the Dem primary to Cuomo, her name will be on the WFP ballot line in November, and she has pledged to actively campaign.

Or maybe not. The Post reports that a WFP official says the party would not play spoiler in the fall if Cuomo wins the primary.

That development might make de Blasio’s decision easier. He could endorse Nixon for the primary and, under the cover of the WFP, back Cuomo in the fall, which would reduce the damage to his standing nationally.

Hank Sheinkopf, a veteran strategist, thinks it’s possible that one reason de Blasio is raising the profile of wife Chirlane McCray is that McCray might first endorse Nixon to test the impact.

“Chirlane could endorse, and the argument would be that she could help get African-Americans and women to go for Nixon,” Sheinkopf says.

In fact, the women’s vote could be key in the primary, with New York never having had a female governor or major party nominee. The #MeToo movement might give Cuomo something to worry about.

Already, he’s trying to find the right tone to counter Nixon’s harsh attacks without looking heavy-handed. She has accused him of being a closet Republican, a bully and an enemy of public schools, and he’s ignored her or let aides respond.

That won’t work forever. If she keeps rising in the polls, he’ll have to counterattack. And he’ll probably have to debate her before the September primary, so how he handles a confrontation will be important.

As for the mayor, he is trying to suggest he’s above it all, and has kept his cool publicly even as Cuomo has used state powers to grab de Blasio’s turf. While the feud started as a personal grudge match, it’s now about which New Yorker gets to define what it means to be a progressive for 2020.

De Blasio never winced when Cuomo openly tried to find someone prominent to challenge him in last year’s mayoral primary, but nobody took the bait. While his stealth support for Nixon is bold payback, soon enough, the mayor must either go all in for her — or fold his cards.

Before he does, he might recall that no governor in memory has lost a primary, and New York is such a blue state that Cuomo is more likely than not to win a third term.

Given that history and the ability of a governor to punish the city, Sheinkopf thinks de Blasio already is in the danger zone.

“These things never work out well for mayors,” he says, adding a final warning: “If you’re going to take on the king, you better kill him.”

Words to the wise, no matter the source:

“What I have been telling all reporters is that many stories about our investigation have been inaccurate,” a spokesman for special counsel Robert Mueller told the Daily Caller website. “Be very cautious about any source that claims to have knowledge about our investigation and dig deep into what they claim before reporting on it. If another outlet reports something, don’t run with it unless you have your own sourcing to back it up.”

The topic in question was a McClatchy News report that Mueller had evidence that lawyer Michael Cohen, despite denials, had in fact traveled to Prague in the summer of 2016. That was a key claim in the Russian dossier report, which had Cohen meeting secretly with Russian oligarchs to help Trump defeat Hillary Clinton.

In fact, the warning to reporters against trusting anonymous sources in other outlets is Journalism 101, but the collapse of media standards makes it sound like something startlingly new in 2018. That the warning came not from an editor but from an anonymous government flak should embarrass the media world.

Yeah, right. The only thing that will embarrass Big Media is if Trump survives their attempt to destroy him.

Reader Edwin Schindler voices a worry I hear often these days. He writes, “If the losers are unwilling to accept the results of elections and work within the democratic system toward the next election, then the alternative taught by human history is violence.

“Elections, and the acceptance of the outcome, is the only alternative to rule by force.”

Headline from The Mirror: Robot running for mayor in Japan, promising “fairness and balance” for all.

That’s the problem with robots. Humans know the whole point of politics is to screw the other side.