How do you preach to the choir in a documentary? Especially if the title of the film — “Inequality for All” — is in your face?

Al Gore tried it with “An Inconvenient Truth” and produced an Oscar-winning success about global warming. But can Robert Reich repeat the pattern with a similarly urgent message about the growing income gap?

To Reich, the middle class is sacred: the single most important reason for prosperity in boom times. If it’s shut out of the process, if kids can’t get student loans without facing a lifetime of debt, if farms can’t show a profit but chief executive officers become obscenely rich, something’s very wrong.

If you watch a lot of cable news, you’ve seen Reich, who was a rabble-rousing secretary of labor in the Clinton administration, presenting this theory. But how do you engage and hold the attention of an audience that is already saturated with apocalyptic statistics?

Fortunately, “Inequality for All” is the work of entertainers who aren’t above using Dolly Parton’s “9 to 5” on the soundtrack or throwing up brightly animated charts to demonstrate how “The Great Prosperity” has become “The Great Regression.”

Directed by Jacob Kornbluth (“The Best Thief in the World”), this lively film takes a “greatest-hits” approach to Reich, who fears that we’re headed for another financial debacle if we don’t take a more serious approach to taxes and education.

In a craftily edited showdown with talk-show hosts Lou Dobbs and Bill O’Reilly, it’s Reich’s eternal optimism that triumphs. Even if the confrontation includes no invitations to repeat visits from Reich.

John Hartl: johnhartl@yahoo.com