As with Voltaire's Holy Roman Empire, the Socialist Workers Party negates the meaning of every word in its title. It never had much to do with the workers: like a minor public school, the SWP is a home for dim, middle-class children. For years it has been a sect or cult rather than a party - think of the Moonies, but without the smiles. Now it is giving up on socialism to form an alliance with Islamic fundamentalism.

The enemies of political freedom and the enemies of religious and sexual freedom are at one, and will soon be presenting joint candidates to the electorate. Party allies are feeling the shock of the SWP's opportunism. Unknown to most of its members, CND is having a huge internal row between Marxist-Leninists and the rest about whether it should abandon its political neutrality and endorse the Stop the War Coalition. The coalition itself and the Socialist Alliance, a grouping of far-left parties, of which the SWP is one, are in crisis. In theory, all three organisations should have been enjoying a collective moment of triumph. On 15 February, the Stop the War Coalition organised one of the biggest demonstrations in British history. According to the police, 750,000 people marched. According to the coalition, the true figure was two million, although, as we shall see, there are reasons for believing that the claim is tosh. I was astonished, and remain astonished, that a section of right-thinking progressive opinion, which bridles at the smallest sign of sexism, racism or human rights abuse, allowed itself to be organised by the friends of tyranny. Andrew Murray, the coalition's communist chairman, celebrated Stalin's 120th birthday and condemned the 'hack propagandists' who went on so about the millions of slaughtered innocents. George Galloway, its star speaker, saluted Saddam Hussein's 'courage' and no one in the coalition told him that a fascist dictator didn't require a great deal of courage to order, for instance, the gang rape and mutilation of a young political opponent and the delivery of a video of the crime to her powerless family. (In these circumstances, the courage is all on the other side.) The SWP was the driving force behind the coalition, but it wasn't well suited to coalition politics. It is a Leninist party which believes in the overthrow of democracy and the establishment of a dictatorship of a proletariat that will be following its guidance.

As a democratic socialist and a supporter of the Kurdish and Iraqi opposition to Saddam Hussein, I know that I tend to be intemperate when discussing these creeps. In the interests of balance, I should allow Mike Marqusee, press spokesman for the Stop the War Coalition and leading member of the Socialist Alliance, to have his say. Unlike many of his comrades he talks in sentences rather than Dalek-like slogans. His books on the meshing of politics and cricket have made him a worthy successor to C L R James. In short, he's the best and the brightest the coalition has to offer.

In a speech delivered to a seminar organised by the Signs of the Times socialist forum, he explained what working with the SWP was like. 'The SWP is constitutionally incapable of working with others on an equal, honest and transparent basis. In the end, their aim is dominance, and anything that threatens or undermines that dominance will always, in their eyes, be suspect . . . Truth is reified in the form of jargon - and any nuance that cannot be expressed in that jargon is ruled out of consideration. In the end, the SWP is imbued with an authoritarian ethic - most recently confirmed by their readiness to dub as 'divisive' or 'disruptive' anyone who voices political preferences contrary to theirs.'

Marqusee cited what happened at a 'crudely packed meeting in Birmingham' where one of the few 'genuinely independent (and respected) trade union activists the SA could boast' was forced out. There were other instances he might have mentioned, but his most interesting evidence was of Stop the War's determination to support the British cheerleaders of Ba'athist tyranny above all else. Marqusee explained that the coalition had been quiet about Tony Blair's embarrassment at the failure to find chemical and biological weapons because the SWP's priority was 'campaigning for George Galloway', who is merely threatened with the loss of the Labour nomination in Glasgow rather than, to cite a second Iraqi example, being fed feet first into a plastic-shredding machine like the opponents of the 'courageous' Saddam.

Marqusee concluded with a picture of an SWP whose dedication to spin and control surpasses new Labour's. 'Instead of sober assessment of our success and failures, strengths and weaknesses, we're offered empty boosterism. The numbers attending meetings or demos are routinely inflated there go the two million marchers, then and the complexity of multi-faceted developments is unacknowledged. What has disturbed me most in working with the SWP has been their flagrant ethical relativism. This is an ancient foible of the left - a conviction that the class struggle, or the building of the revolutionary party, or the sheer evil of the forces we find ourselves up against justifies any behaviour, no matter how dishonest, duplicitous, or destructive to others. In their competition with the rest of the left, in their drive to maintain control (including control of their own members), anything goes.'

Indeed it does, including alliances with feudal theocracy.

To understand how the Stop the War Coalition has ended up in bed with the forces of reaction, put yourself into the mind of Lindsey German and other leaders of the 3,000-strong party.

You spend your life waiting for a revolution that never comes. The proletariat, whose vanguard you have appointed yourself to be, refuses to follow your instructions. Your voice grows hoarse screaming in rooms filled with, at most, a few hundred supporters. And then, on a bright February day in 2003, you find yourself declaiming to hundreds of thousands in Hyde Park. (Millions, according to your spin-doctors.) It's bound to go to your head, even if the masses haven't the faintest idea who you are and can't work out why they're not being addressed by someone they've seen on the telly.

And then it all goes wrong. Having organised an unprecedented protest, you take the pressure off Blair and make a nonsense of your own slogan by announcing that the next demonstration won't be until after the war has started. (I used the word 'dim' to describe the SWP leadership for good reason.) You can't admit the mistake; instead you concentrate your hopes on the masses going home from Hyde Park and voting for you in the May elections. The great day arrives and the Socialist Alliance is defeated in every ward except one: it wins a seat on Preston City Council, not previously noted as a hotbed of revolutionary socialism. The Socialist Alliance took the seat because 12 local mosques put their support behind its candidate.

You mull on the reasons for your success, and reflect that socialism with priestly allies isn't new for you. The Stop the War Coalition had, after all, invited the Muslim Association of Britain to co-host its demonstrations. The association is an offshoot of the Muslim Brotherhood. The Brotherhood was founded in 1928 by Hassan al-Banna, who admired Mussolini's blackshirts. Its website proclaims: 'Qu'ran is our law. Jihad is our way. Dying in the way of Allah is our highest hope.' At Stop the War demonstrations last autumn, association members handed out leaflets which explained that the punishments for a Muslim who freely decided to abandon his or her faith should include death. Tricky situations have had to be negotiated since. At a Stop the War meeting in Birmingham, the clerics and their supporters instructed Asian women to sit separately from the men. Iranian socialists had to be shut up when they protested that they knew from bitter experience where religious bigotry led. Why not win more than a seat in Preston by seeking an arrangement with the only people who can deliver the votes?

Which is what the SWP is trying to do. It is planning to join with Muslim activists and mosques in Birmingham to run 'Peace and Justice' candidates in the 2004 European elections. Complaints from queasy SWP members who worry about women's and gay rights were dismissed by Lindsey German at this summer's SWP conference. 'I'm in favour of defending gay rights,' she declared. 'But I am not prepared to have it as a shibboleth, created by people who . . . regard the state of Israel as somehow a viable presence.' (Both the SWP and the Muslim Association want to abolish Israel.) Common ground is already being found, and the latest issue of Socialist Worker is calling on the comrades to protest against women who lap-dance in Spearmint Rhino strip clubs.

I doubt the tactic will work. If conservative West Midlands Muslims can win seats, why shouldn't they drop the Trots and do it on their own terms? If the coalition goes ahead, we can look forward to the delicious spectacle of new Labour, Lib Dem and even Tory candidates attacking the SWP from the left on nearly every issue in the political alphabet from abortion onwards.

The story does, however, contain two wider points. First the SWP is exhibiting in extreme form that strain of liberal opinion which said after 11 September that fundamentalism was really the fault of the west and fundamentalists deserved to be understood. Second, and this really should have been obvious decades ago: the democratic left will never get anywhere in this country unless it makes a clean break with the supporters of dictatorship, whether they come in clerical or political guise.