Tuesday is Primary Day.

Polls are open in the city from 6 a.m. to 9 p.m.

In the Democratic primary, Mayor Bill de Blasio faces off against some long-shot challengers — former Brooklyn Councilman Sal Albanese, police-reform activist Robert Gangi, Brooklyn attorney Richard Bashner and tech entrepreneur Mike Tolkin.

The most-watched City Council race is in Queens’ 21st District, where ex-con Hiram Monserrate, a former state senator and councilman, is attempting a comeback against Assemblyman Franciso Moya, who has the support of the Democratic Party.

In Brooklyn, acting DA Eric Gonzalez is running against five Democratic opponents — term-limited Councilman Vincent Gentile and four candidates who have worked in the DA’s office: Ama Dwimoh, Patricia Gatling, Marc Fliedner and Anne Swern.