Michael Moore began his filmmaking career with an investigative passion and a flair for showmanship. His 1989 debut feature, “Roger & Me,” used the funny populist hook of Mr. Moore, in his working-class Everyman persona, pursuing Roger B. Smith, the then-head of General Motors, to confront him about the company’s abandonment of Mr. Moore’s hometown, Flint, Mich.

In his new documentary, “Fahrenheit 11/9,” Mr. Moore, his sense of showmanship not only intact but enhanced by a bigger production budget, fills a water truck with the same supply that’s piped to Flint residents for drinking and bathing.

This water is famously polluted with lead (among other toxins), in a yearslong scandal that’s still not come to a satisfactory conclusion. (“No terrorist organization has figured out how to poison an entire American city,” Mr. Moore notes in voice-over. “It took the Michigan Republican Party to pull that off.”) He drives the truck to the home of the most powerful Republican in Michigan, Gov. Rick Snyder, and waters his lawn with it.

For Mr. Moore, the situation in Flint is a microcosm of the disaster he sees President Trump imposing on this country.