White House? He can’t even keep Gracie Mansion afloat.

Not only are president wannabe Mayor de Blasio’s poll numbers underwater with state voters — he’s also their least-liked Democrat in the potential field of 2020 contenders, a survey revealed Thursday.

Only 24 percent of New York state voters had a favorable opinion of the mayor, while 49 percent viewed him unfavorably, the Quinnipiac University poll found.

De Blasio was also in political trouble in the city he has led for more than five years — just 36 percent of Big Apple voters rated him favorably compared with 48 percent unfavorably.

The mayor is too far left, even for many of New York’s Democrats, said pollster Doug Schoen.

“For those who are not socialists, Bill de Blasio isn’t what they’re thinking about in a presidential candidate,” he said.

Even among Democrats, Hizzoner couldn’t break into positive territory — just 36 percent viewed him favorably versus 38 percent who gave him a thumbs-down.

The only 2020 candidate more toxic to state voters was President Trump — 67 percent held an unfavorable of view of him compared with 28 percent who liked him.

Topping the poll was former Vice President Joe Biden, at 62 percent favorable and 24 percent unfavorable, followed by Sen. Bernie Sanders, at 51 percent and 38 percent.

Noting de Blasio’s poor showing, another political expert said the mayor likely did himself no favors by gallivanting around the country flirting with a White House run.

“New Yorkers have a long history of warm regard for mayors like La Guardia and Bloomberg, who see [the mayoralty] as a 24/7 mission, and that’s not in de Blasio’s personal job description, it seems,” said political strategist Gerry O’Brien.

“Some voters have a perception that he has an up-at-the-crack-of-noon attitude. He’s often late to events, and that is part of what you see reflected in that Q poll.”

While he hasn’t announced a 2020 run, de Blasio has made trips to the early-voting states of Iowa, New Hampshire and South Carolina in recent weeks. He is also heading to Washington, DC, this weekend to appear at the American Israel Public Affairs Committee conference Monday and Tuesday.

“My advice would be to take the subway to all parts of the city instead of going to Iowa, New Hampshire and South Carolina,” Schoen said.

Plus, city voters are likely feeling de Blasio burnout, added veteran political consultant George Arzt.

“There’s voter fatigue with de Blasio,” he said. “It’s a very tough job with a lot of negatives. There’s never a mayor in a second or third term who didn’t have a lot of negatives.”

Still, de Blasio brushed aside his dismal poll numbers Thursday.

“I will do what I think is right. I don’t have decisions based on polls,” he said.

Asked whether he feels like he could even win New York City, he reminded reporters how badly he was trailing when he ran for public advocate and during his first mayoral campaign.

“It is not where you start, it is where you finish,” he said, using a line he has repeated over the weeks.

De Blasio wasn’t the only New York candidate getting rough treatment from the home crowd.

Sen. Kirsten Gillibrand, who has officially entered the race, was viewed favorably by only 29 percent of voters, while 35 percent viewed her unfavorably with the rest undecided.

But the assessment was not as damning for New York’s junior senator, O’Brien said.

“Gillibrand is one of 100 senators,” he said. “It’s easier for someone like that to escape blame — there’s only one mayor of New York City for good or bad.”

Meanwhile, one New Yorker offered her own verdict on de Blasio’s presidential chances.

As the mayor was riding the subway to an event in East Harlem, a rider hollered, “Forget 2020!”

Quinnipiac surveyed 1,216 New York state voters from March 13 to March 18. The poll has a margin of error of 3.8 percentage points.