Blame it on Mort the Mouth: according to this bewilderingly nostalgic documentary, the current nadir of American public discourse can be traced to Morton Downey Jr., the bombastic conservative broadcaster whose syndicated TV show was short-lived (1987-’89) but influential. Downey was confrontational, profane, unapologetically sexist, and sometimes physically abusive, much to the delight of his rabid studio audience, which New York’s then mayor Ed Koch called “a lynch mob.” Assorted celebrities and Downey’s producers offer some insights, but the movie dwells too long on a group of worshipful former audience members—presumably because filmmakers Seth Kramer, Daniel A. Miller, and Jeremy Newberger were once fanboys themselves.