A leading good government advocate is criticizing Mayor de Blasio for siding with Republicans in opposing a state constitutional convention.

In November, voters will be asked to decide whether the state should hold a convention to make changes to the state constitution.

By law, the convention can only be held every 20 years.

Dick Dadey, executive director of the good-government group Citizens Union, supports the convention because it could lead to more competitive elections and urged de Blasio to get on board in a letter delivered Tuesday.

In it, Dadey cites de Blasio’s statement that a convention is “not worth the risk” and points out that he’s aligned himself with the state Republican and Conservative parties, as well as Right-to-Life groups and the state Rifle and Pistol Association.

“The question is why these groups agree with you that a convention is not worth the risk?” Dadey writes.

“I hope you take more time to study this issue and think about why these anti-progressive groups so vehemently oppose a constitutional convention.”

De Blasio said in April he opposes a convention because it would be “dominated by moneyed interests.”

“I’m very, very concerned about any outcome that would come from that,” he added.

Other groups that oppose a convention include the AFL-CIO and the city and state teachers’ unions.