RICHMOND — Former president Barack Obama took the unusual step of criticizing and mocking Virginia’s Republican gubernatorial candidate for a controversial ad blitz highlighting Latino gang violence. As he rallied for the Democratic candidate Ralph Northam on Thursday night, Obama mentioned that he and his wife, Michelle Obama, have seen Republican Ed Gillespie’s commercials airing on television in their D.C. home. The spots say Northam is “weak on MS-13” as images of tattooed Latino men and the gang motto “Kill, Rape, Control” pop up on the screen.