MIAMI, Fla — Mayor Bill de Blasio managed to bungle Wednesday night’s debate before it even started.

De Blasio’s upstart presidential campaign introduced labor leader Stuart Appelbaum as one of his supporters preceding the first Democratic Presidential Debate in Miami Wednesday night — but Appelbaum said he wasn’t backing Hizzoner.

“I’m not a surrogate for anyone,” the labor leader told reporters gathered for the 2020 Dems’ debate. “I’m here because of my role within the Democratic National Committee.”

Appelbaum — head of the 100,000-member Retail, Wholesale and Department Store Union — was one of two people the mayor’s campaign brought to the press-filing center to talk up de Blasio’s record just two hours before the debate.

The other pseudo-surrogate was Bronx state Sen. Luis Sepulveda, who appeared to be fully on board with the de Blasio campaign.

But Appelbaum was clear that he wasn’t serving as a proxy for the mayor and his union has not made an endorsement.

The Democratic operative said he was impressed with a host of other candidates who are not de Blasio — including Massachusetts Sen. Elizabeth Warren, New York Sen. Kirsten Gillibrand, Vermont Sen. Bernie Sanders and former vice president Joe Biden.

Appelbaum did praise de Blasio for being the city’s most “pro-union, pro-worker mayor” and cited his leadership on the $15 minimum wage and Universal Pre-K.