The Labour party faces the greatest crisis in its history. Momentum – a party within the party which is dedicated to moving Labour to the far left of the political spectrum – is on the point of winning control of Labour’s policy, programme and constitution. Momentum members and supporters – 844 of the 1,135 delegates – dominated this year’s annual conference and it seems likely that elections, now taking place, will provide a clear Momentum majority on Labour’s national executive.

Momentum does not disguise how it will use the power which domination of the party machine guarantees. The shift to the left will begin with a revision of Labour rules and continue with the replacement of moderate MPs and councillors with Momentum nominees. The threat to Labour’s future is real and obvious. Yet, not one of those who resigned from the shadow cabinet in protest against Jeremy Corbyn’s leadership, has publicly condemned what amounts to a takeover.

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In almost every constituency Labour party, there are men and women who stand ready to challenge the subversion. Indeed, many of them are already fighting lonely battles. They need politicians of principle to defend their aims and values and make plain that moderate democratic socialists have a clear vision of the better Britain which they hope to build. Thirty years ago, moderates won the battle against Militant by taking the campaign to the country and demonstrating that genuine democratic socialism was worth fighting for. Now Momentum is winning by default. “Real Labour” needs a better response to the prospect of a far-left takeover than glum opposition to Len McCluskey and all his works – understandable though that is.

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Politicians have a duty to defend their beliefs. Democratic socialism will not succeed, and may even not survive, unless it protects its ideological frontiers. The doctrine “no enemies to the left” is a prescription for Labour’s transformation into a different party. What amounts to an invasion should have been resisted from the moment that Jeremy Corbyn signalled that what was once called infiltration had become recruitment. And there is the more immediate problem of the next general election. Labour cannot win while it is associated with extremism. If the extremists begin to deselect moderate MPs, others – who are, or believe themselves to be, under threat – will split the party and keep Labour out of office for a generation.

I want Labour to form the next government but Jeremy Corbyn reduces the chance of victory by welcoming, as bona fide party members, politically disreputable opportunists. Fears about victory for the far-left helps to hold down Labour’s opinion poll lead to 4-5% at a time when the government’s incompetence should put him 20 points ahead. Some of Jeremy Corbyn’s advisers may be careless about his election prospects, because they do not believe in parliamentary government. But whatever its origins, if he will not remove the millstone from round his neck, the party must do it for him.

The cull of councillors has already begun. The assault is most violent in the London boroughs in which Momentum thrives. Last month, it moved though Haringey, ward by ward, claiming to be Corbyn’s revolutionary guard – although his leadership is no longer in question – and pretending that it stands for democracy when it really seeks Labour’s domination by a narrow clique.

Facebook Twitter Pinterest Jeremy Corbyn takes a selfie with Labour MPs in Westminster Hall at the Houses of Parliament in central London on 28 June. Photograph: Jack Taylor/AFP/Getty Images

The Haringey meetings were swamped by recent recruits. Many of them came to selection conferences with the premeditated intention of sacking the sitting councillors. The same overt hostility poisoned the atmosphere at the annual meeting of the Wavertree constituency in Liverpool, where Momentum won nine out of 10 of the influential offices. Long-standing party members described the atmosphere as “intimidating”. Several of the aggressive newcomers were identified as recent pamphleteers for the far left.

Momentum’s success varies from place to place. In Manchester and Sheffield, moderate councillors have already been deselected or have chosen to resign rather that face the humiliation of rejection. In Liverpool, the old gang is back. All that has changed is that the Militants now travel to meetings with bus passes. In fact, the Labour party is in far more danger than it was in the 1970s. Militant was better organised than Momentum but commanded less support and was active in fewer constituencies. Most important, Militant had no trade union backing. Momentum has Unite and its voluble general secretary, Len McCluskey. In consequence, an afternoon of telephone calls identified six members of parliament who anticipate de-selection. One forecast “a bloodbath”.

Perhaps the fightback has not begun because the one-time champions of Real Labour fear they cannot win. But, while they will certainly lose if they keep their heads below the parapet, Momentum can be beaten. It was beaten in Lewisham East, where Heidi Alexander MP reclaimed her local party by dint of personality and hard work. It was beaten in the battle over Sheffield’s local authority budget by Julie Dore, the council leader, who publicly challenged the claim that political purity demanded refusal to implement government cuts and the surrender of services to direct Whitehall control. Labour First – Real Labour’s only lasting attempt to keep like-minded socialists in touch with each other – continues to identify the true allegiance of candidates for party office. But a national campaign to rescue Labour from extremism requires public leadership – loud, persistent and unafraid.

The shadow cabinet rebels of 2015 can only provide it if they make the admission that they were wrong to say, as I did, that Jeremy Corbyn could not lead Labour to victory. Then they can express the pleasure with which they acknowledge their mistake. After that, they can spend their evenings in cold halls, speaking to small audiences about Real Labour’s true values. They will be accused of harbouring the intolerant hope of excluding from the party everyone with whom they disagree – a strange allegation to be made by people who are actually managing a purge. It will be neither as comfortable nor as glamorous an occupation as asking clever questions in the House of Commons or appearing on television –but it might just save British social democracy from extinction.