Mayor de Blasio won an endorsement from one of the city’s most powerful unions Thursday — a development that’s sure to give fellow Democrats pause when considering a primary challenge.

The nod from 32BJ SEIU, the service workers union which boasts a membership of 75,000 in the city, comes on the heels of endorsements from the sanitation and retail workers’ unions.

Aside from their symbolic stamps of approval, all three give de Blasio a significant head start against potential opponents.

“He’s rolling out really early to scare off political rivals,” said College of Staten Island Prof. Richard Flanagan. “Yeah — lock [endorsements] up early and hope the scandals dry up.”

De Blasio is still facing several probes, including a federal investigation into his fundraising operations.

On Thursday, he said he sleeps “very well at night” when it comes to the investigations.

“I don’t sit around waiting around for a surprise I don’t think exists because I believe everything was done properly,” he said.

Aggressive primary challengers also don’t seem to a source of much worry.

Hector Figueroa, 32BJ’s president, noted that de Blasio was the only candidate to reach out to the union for its endorsement.

“Nobody [else] has come to us,” he said. “Nobody has said to us — 32BJ, we would like you to consider us to be a candidate for mayor.”