The Sweden Democrats, a populist anti-immigrant party, has had its image transformed in a couple of days – from well-coiffed gentlemen in suits to hooligans wielding scaffolding poles.

This week, the evening tabloid Expressen, which has kept a close watch on the Sweden Democrats since the party entered parliament two years ago, published a video that was taken on a street in central Stockholm two years ago after a long evening's celebration. It shows the party's spokesman for economic policy, and an MP, Erik Almqvist, having a full and frank exchange of views with a Swedish stand-up comic of Kurdish extraction.

Almqvist abuses him racially, says: "No, this isn't your country – it's my country," "Don't fuck with Swedes," and "You're arguing like a little cunt." He's with another of the Sweden Democrats' most prominent public figures: the video was filmed by the party's justice spokesman, Kent Ekeroth. Both men loom threateningly in front of a drunk man who comes by. When a young woman objects to this, Almqvist abuses her by referring to her as "a little whore". Kent Ekeroth also pushes another young woman against a car.

Later the video shows how both MPs arm themselves with scaffolding poles from a building site, but before they can set out to find the drunk again they hear police sirens. They put the poles away, straighten their suits and tell the police that the man was after them and they had better arrest him. Then they turn away, giggling, and Almqvist says: "It always pays to act respectful."

The party has nearly doubled its support since it entered the Swedish parliament two years ago. In the most recent polls the Sweden Democrats are the country's third-biggest party, which is shocking in a country that managed – unlike much of the rest of Europe – to exclude anti-immigrant parties for many years.

The party leader, Jimmy Akesson, has largely succeeded in cleaning up the party's reputation as far-right extremists, not least by making short shrift of party members who have expressed openly racist views. Recently he announced a policy of zero tolerance towards racists in the party.

This was an easy policy to maintain so long as it was only relatively unknown members on the fringes of the party who disgraced themselves. But now the party has been hit by its worst crisis ever – and all because of a film that one of its own leaders took with his mobile camera. The facade so carefully erected of a respectable party has suddenly collapsed. The leadership has taken drastic steps to deal with the crisis quickly.

Erik Almqvist has been relieved of all his responsibilities, and Kent Ekeroth, who made the film, has been told to take "time out". But this may not be enough. Anonymous sources have told two of Sweden's biggest newspapers that the party leader knew of the video himself. A version had been published on the Sweden Democrats' own YouTube channel two years ago, though with all the embarrassing bits edited out. The question now is whether the leader knew about the uncut version.

The racism, sexism, and propensity to violence that two of his closest colleagues display will be difficult for him to distance himself from. The process of normalising the party has had its gravest setback to date. All talk about how the party is merely "critical of immigration" rather than xenophobic collapses when you see the video, which half a million Swedes have done so far.

The irony is that the video was made to be used as evidence in case the two MPs were attacked. Instead, it ended up, uncut, in the hands of a newspaper – after everyone involved had denied that they said or did the things they are shown saying and doing. They now stand exposed as the clumsiest politicians in Swedish history. If the film weren't so nasty, the whole country would be laughing them into oblivion.