Mayoral candidate Bill de Blasio will actively campaign to help elect fellow Democrat Ken Thompson as Brooklyn District Attorney to thwart a comeback bid by incumbent Charles (Joe) Hynes, The Post has learned.

Thompson defeated the 24-year  DA in last month’s Democratic primary.

The Post disclosed this week that Hynes has decided to keep running on the Republican and Conservative lines in the Nov. 5 general election.

De Blasio had also backed Thompson in the primary.