Gore Vidal: The United States of Amnesia opens with the novelist/essayist/playwright and screenwriter standing over his own grave. He is pointing out where his year of death will be etched, and he has apparently accepted that it will be written soon: He leans heavily on a cane, moves only with Olympian effort and has become exhausted with the daily ordeal of living. But if he has accepted his death – which would come to Vidal at the age of 86 in 2012 – he has not accepted what has become of his country. To him, it is a republic twisted into a decadently gnarled, bloody-minded empire. Asked if he can imagine what legacy he will leave when he dies, Vidal snarls like a mortally wounded wolf: "I couldn't care less."

It's vintage Vidal – funny, fierce, thoroughly unsentimental and consistent with a life spent playing himself in the public eye. Among the last and greatest of that virtually extinct species of talking head known as the public intellectual, Vidal was a presence in American life throughout the last half of the last century. A hard-liberal blend of Groucho Marx, Noam Chomsky and Oscar Wilde, he marshalled his considerable TVQ in the cause of compelling America to face up to the reality of its own cynical betrayal by power, money and rampaging self-interest.

First-time feature filmmaker Nicholas D. Wrathall's portrait of the wolf in winter may err on the side of a certain formal conservatism. But if you're going to make a standard talking-heads documentary, you could do worse than make it about one of the most charismatically eloquent talking heads ever to lock eyes with a camera lens. In his way, Vidal was as savvy a student of the effects of mass media as were Marshall McLuhan, Johnny Carson and Vidal's ambivalently admired John F. Kennedy. And for anyone who grew up TV-transfixed in the sixties or seventies, Vidal was as staple a tube fixture as talk shows, political turmoil and war in Southeast Asia.

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While Gore Vidal: United States of Amnesia provides much by way of biography and context for Vidal – he was born wealthy into a political dynasty – the real subject here is Vidal as media phenomenon. A kind of floating one-man guerrilla message system, he knew exactly how to use his considerable appeal as a camera subject to conduct a relentless war of words against the power elite into which he was born. (Watching his titanic televisual dust-up with William F. Buckley during the 1968 presidential campaign is like being reminded of a time before electricity: You mean they actually let smart people debate at length on network TV?)

When Vidal speaks of Kennedy, a friend through family connections, who charmed him more than anyone else he'd ever met – and who inspired him into his own unsuccessful runs for office – he reserves a special kind of wary contempt. In the Camelot myth of the martyred, movie-star peacemaker, Vidal obviously sees his equal in manipulated pop-culture seductiveness. He both marvels at and mourns the persistent idea that Kennedy might have ended the fiasco in Vietnam.

But he saw in that idea something galvanizing to his own ideological calling: It was by falling for Kennedy's image that Vidal first comprehended the role of corporate ad-man baloney in the bolstering of presidential worthiness in America, and that he subsequently took as his primary mission the calling of it in every subsequent administration.

Indeed, he became convinced that the price of ultimate political power in America was the surrender of convictions. In the big production that was – is – American political process, the Devil was the director. In one of the movie's most indelible moments, Vidal watches Barack Obama's acceptance speech in 2008 with a circumspect detachment. Much as one imagines he'd like to buy the line of a changed America, he knows too much. Small wonder, and even less satisfaction, that he calls these the "four most beautiful words in the English language": I told you so.

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By that time, of course, Vidal had endured something one imagines even he, at his most imperiously cynical, could not have imagined: the two-term presidency of George W. Bush, famously dubbed by Vidal as "the stupidest man in the United States." In truth, however, he had endured more: the marginalizing of his own voice; the decline of public political discourse to hawk-and-spit news-channel bear baiting; and the thoroughly deflating realization that nothing he'd said or done, no matter how brilliant, illuminating, urgent or true, had really changed anything.

No wonder he seemed so at home looking down at his own grave. He might not have been happy to die, but he was ready.