When Gore Vidal declared in an old television debate with William F. Buckley Jr. that 5 percent of Americans had 20 percent of the income and the bottom 20 percent had 5 percent, he was raising an alarm. That observation may be the most shocking moment in “Gore Vidal: The United States of Amnesia,” Nicholas Wrathall’s admiring documentary portrait of Vidal, who died in 2012 at 86.

Why shocking? It illustrates the astounding degree to which perceptions have changed over time. By the standards of today, when income inequality has widened exponentially and the middle class is shrinking, statistics that infuriated Vidal sound like the answer to a socialist’s prayer.

Intellectual celebrities nowadays eschew the lofty, disdainful tones affected by Vidal and Buckley, his conservative opponent, who died in 2008. Public discourse is louder, angrier and coarser. No liberal of comparable eloquence has taken Vidal’s place in the public square, although Christopher Hitchens, who died in 2011, deemed himself Vidal’s heir apparent, until the two men had a falling out.

Heavily seasoned with epigrams worthy of Oscar Wilde, this entertaining documentary portrays Vidal as a pessimistic political prophet with streaks of paranoia and misanthropy, but a truth teller nonetheless. In carefully selected excerpts from interviews and conversations conducted during all phases of his career, he exudes a patrician hauteur fed by a reservoir of chilly rage. For this champion of democracy and freedom of speech was an aristocrat who blamed many of America’s blunders on the short memory, or “amnesia” span, of the body politic.