For its first third, Nick Broomfield's documentary feels worthy of a toast by the Tea Party. Our bumbling Brit, gamely kitted out in red-check lumberwear, heads to Wasilla, Alaska, where he's charmed by Sarah Palin's parents – plus Chuck, their antler-sucking puppy – and nods politely at gasps of adoration from wide-eyed Palin pals.

He and his subject even seem to hit it off when Broomfield requests an interview at a book-signing: "You betcha!" is her dazzle-grinned reply.

Such apparent eagerness to be amiable only makes the last hour the more devastating. As Broomfield encounters more and more local people, and his requests for interviews are batted away with increasing energy, so the picture of a woman who appears to have made more enemies that most of us have seen episodes of the Sopranos sharpens into focus.

One after another classmates and relatives come forward to grind axes and shiver at the prospect of a Palin presidency. They're joined by former campaign managers, chief strategists, PR agents, mentors, preachers and policemen, all singing from the same hymn sheet: Palin's ruthlessness and venom for revenge knows little limit.

Nick Broomfield discusses Sarah Palin - You Betcha! guardian.co.uk

You Betcha! works too as a portrait of how place can produce personalities: Wasilla is a defensive, isolated community where even Palin's detractors vote Republican, wear stars and stripes braces, and keep their beauty queen tiaras wrapped carefully in cake tins. It's a city where prom-queen politics can bleed far into adulthood, and one man's theory about how Palin still acts like "the most popular pre-teen girl" rings right when you hear her giggle at enemies being insulted, or watch her chew gum. It's the stuff of high school horror movies: can you stop the most popular girl in school from stabbing you in the back?

One former friend speaks of such devotion among acolytes that they'd "jump off the side of the planet – if it were flat – like lemmings". It's a statement that subtly suggest the community's general scepticism in science, and Broomfield unearths compelling stuff about the role Palin's religious zeal may play in her politics and confidence; the local pastor suggests Palin believes she's a chosen one – and that humans and dinosaurs once co-existed.

Broomfield's film has had some of its edge sanded off by the fact its target is now, effectively, neutralised, superseded by a younger model, and so discredited her getting her mitts on the keys to the White House feels a pipe dream, rather than an imminent nightmare. There's not a lot that's truly new here, and Broomfield must bulk his testimonies out with file clips. As cuttings jobs go, though, it's razor sharp. He may look like a pussycat, but Broomfield, too, is a pitbull within.