★★★★

Of all the morality tales to emerge from the advent of social media, the most salutary – and bleakly comic – is that of the unfortunately named New York politician Anthony Weiner, who allowed photos of his erect penis to find their way onto his Twitter feed.

He did it once. Then he did it again three years ago – just as his political comeback, a campaign to become New York mayor, was gaining momentum.

This documentary, a winner at this year's Sundance Festival, is his story and it's a terrific film. Directors Josh Kriegman and Elyse Steinberg have delivered something much richer than the profile of a compulsive sleaze. They've put together an exemplary study in self-delusion. Weiner is afflicted with the misapprehension that he can talk his way out of anything. Donald Trump is a fellow sufferer. The difference is that Weiner is a liberal whose wife, Huma Abedin, happens to be Hillary Clinton's most trusted aide.

Abedin is in the film, occasionally trying to curb her husband's desire to tell all whenever anybody asks. More often, she's at the side of the frame, mute, hurting and clearly wishing he would just shut up. But he doesn't. Only twice does he want the cameras to be turned off – probably because he can see the prospect of divorce looming. Why does she stay? That the mystery remains is the film's only flaw.

Before his habit of "sexting" female fans came to light, Weiner was a young Congressman on the rise. Wiry and hyper-active, he could summon up indignation on demand. Supporters declared him a passionate fighter on behalf of the middle-class voter. He had a message to deliver – which is why, he says, he consented to the documentary. His career was in recovery and he had things to say. A more likely reason is that he can't say no to anything involving a camera.

He has a temper, giving the finger to the media when pushed and refusing to walk away from a damaging set-to with a street heckler, but he also has an uncanny ability to maintain his affability in the most dire circumstances. At times, the filmmakers seem to feel sorry for him, marvelling at the fact that they're allowed to go on shooting after the second scandal breaks.

The nadir comes during the humiliating conclusion of his mayoral campaign, when one of the recipients of his "sexts" tries to crash the post-election party with her own film crew. His efforts to evade her, with Huma in tow, are excruciating – yet typical. He says at one point that the characteristics that got him into trouble also help him to cope with it. Maybe. In the end, it's hard to tell if his downfall is due to blind egotism or suicidal optimism.