Mayor de Blasio took a personal trip to Atlantic City Tuesday for the opening of a health center for a labor union formerly run by his cousin — which was one of his biggest campaign boosters.

The trip had no benefit to the city’s taxpayers, but was a sweet “thank you” to UNITE HERE! — which channeled $175,000 to an anti-Christine Quinn group in 2013.

De Blasio’s cousin John Wilhelm headed the union until late 2012.