It was all going so well. After resigning from Congress in the wake of a cringe-worthy sexting scandal, Anthony Weiner promised to change his ways and successfully transformed himself from tabloid punchline into a viable candidate for mayor of New York City in 2013. That remarkable comeback was supposed to be the subject of the new documentary “Weiner.”

As many of us remember, more of Weiner’s sexual selfies emerged, re-humiliating his wife, Huma Abedin, and sending his campaign into an irreversible death spiral. A moment comes in “Weiner” when the politician watches his failure unfold on television while his wife sits silent as death in the background. The filmmaker Josh Kriegman peeps up, almost inaudibly, to ask: “Why have you let me film this?”

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It’s an excellent question. One answer is that Kriegman, who directed with Elyse Steinberg, worked for the congressman years before the scandal. Another answer is that Weiner, even at his lowest, stubbornly refuses to shrink from public view. By his own admission, the arrogance that allowed him to tweet explicit pictures is precisely what allows him to walk the streets with his head held high.

Kriegman’s unprecedented, unlimited and almost unbearably close access to Weiner makes this one of the best political documentaries you’ll ever see. These are the moments in a candidate’s life you never imagined you’d witness: rehearsing his apology speech for maximum sincerity, discussing truth-stretching strategies before an interview, facing his wife after the latest revelations. Each scene is more jaw-dropping than the last. In one astounding sequence, Weiner literally runs from his past in the form of a publicity-seeking porn star.

You might not vote for the guy, but you have to admire him. Unlike another disgraced New York politician, former Gov. Eliot Spitzer, who broke into flop sweat whenever prostitutes were mentioned in Alex Gibney’s documentary “Client 9,” Weiner owns his mistakes (well, to a point) and never dodges a question. It’s worth noting that Weiner could have shut this movie down at any time; instead, he hardly ever asks the filmmakers to leave a room. What emerges is a fascinating picture of a promising politician undone by hubris. And as the movie suggests, we may not have heard the last of him yet.