T HERESA MAY’S slow progress through the great mangle of Brexit has been so gruesome that it has distracted attention from another political flattening: that of Jeremy Corbyn. The leader of the opposition put in another fumbling performance in the House of Commons this week in proposing that the government should be forced to put off Britain’s departure from the European Union if it doesn’t reach a deal. But lacklustre rhetoric and a feeble grasp of detail mark only the beginning of his problems.

The Labour Party is even more divided over Brexit than the Conservatives. Most Labour members disagree with their party’s official support for leaving, whereas most Tory party members support their party’s position. Mr Corbyn is much farther away from his party’s centre of gravity than Theresa May is from hers. He is a long-standing Eurosceptic who believes that the EU is a capitalist club that stands in the way of building his socialist Jerusalem. He voted against Britain’s membership in 1975, opposed the single market in the 1980s and only pretended to campaign for Remain in the referendum of 2016. He is surrounded by an inner circle of Eurosceptic advisers who do their best to steer a Europhile party in a Eurosceptic direction.

Mr Corbyn has tried to manage these contradictions by resorting to grand banalities. He has claimed that Labour supports a “jobs-first Brexit” that will magically provide all the benefits of Brexit with none of the costs. He has headed off calls for a second referendum by saying that he wants a general election instead. That strategy is wearing thin. With Brexit less than two months away, Mr Corbyn is being forced to make real and urgent decisions. This week he lent his support to Yvette Cooper’s amendment requiring the government to delay Brexit if Parliament hasn’t agreed on a deal by a certain date (the measure failed, in part because Mr Corbyn’s backing was so late and his advocacy so feeble).

Brexit is driving a wedge between Mr Corbyn and his activist fans. Most activists are even more Europhile than the membership in general, particularly the young idealists who flooded into the party from 2015 onwards. Corbynmania is not dead: many activists claim that they forgive their idol his unfortunate views on Europe. But it is hard not to lose some of your enthusiasm when you disagree with your leader on the most important issue of the day. For example, 79% of party members support having another vote on Brexit, whereas Mr Corbyn has done everything in his power to prevent that from happening. Party membership is drifting down and polls show Labour failing to overtake the floundering Tories.

Brexit is wreaking havoc with Mr Corbyn’s plan to turn Labour into a mass movement as well as a parliamentary party. The lefties demonstrating on the streets these days are calling for Britain to remain in the EU, not for the abolition of capitalism. Brexit is also dividing the left. Mr Corbyn rose to power by uniting the broad left against the Blairite right. The 69-year-old looked as if he was a prophet of a progressive future while the middle-aged Blairites looked as if they were locked in a neo-liberal past. Now he is splitting the left between Europhobes and Europhiles (even his long-term ally and shadow chancellor, John McDonnell, is distancing himself from Mr Corbyn’s Euroscepticism) and allowing Blairites such as Ms Cooper to rebrand themselves as champions of an open future.

Above all, Brexit is forcing Mr Corbyn to fight on uncongenial terrain. He takes every opportunity he can to change the topic back to his old favourites: the evils of greedy bosses and the ravages of austerity. The only time he comes alive in prime minister’s questions is when he is talking about victims of the government’s cuts. But his words keep falling on deaf ears.

Mr Corbyn’s response is to shout louder. He is convinced that Brexit is not a “productive antagonism” for the left, as one aide puts it, and that the best way to deal with it is to shift the subject back to the antagonisms that have defined his career. History has other plans. Growing psephological evidence suggests that Brexit is profoundly reshaping British political allegiances. Voters are increasingly defining themselves by where they stand on Brexit rather than by where they stand on old-fashioned politics. Geoff Evans and Florian Scheffner note that only 6% of Britons do not identify with either Leave or Remain, whereas 22% do not identify with a party. Tim Bale, another academic, notes that 61% of Labour members think Brexit is the biggest issue facing the country, versus just 9% who plump for the next-biggest, health and the environment.

The turn of the screw

Mr Corbyn’s contortions over Brexit are forcing his supporters to rethink their idea that he is a man of principle. He seems almost Clintonian in his willingness to triangulate on all things Brexitrelated, embracing vague formulae so he can appease both Leavers and Remainers, and indulging in procedural prevarication in order to avoid making difficult decisions. At the same time, his manifold confusions over Brexit, in interviews and at the dispatch box, are reinforcing his critics’ worries that he is not up to the job of taking real decisions. He often seems to be confused about basic questions such as what a customs union means, let alone the details of complicated negotiations.

The biggest danger for Mr Corbyn is that he will be defined by history as a handmaiden of Brexit if he doesn’t get off the fence and try to prevent it. One prominent Labour Remainer says that he and his friends will do everything in their power to brand Mr Corbyn as a latter-day Ramsay MacDonald, the Labour prime minister who was expelled from his party after he agreed to lead the Conservative-dominated National Government in 1931. Given Mr Corbyn’s irritating habit, throughout his long life in politics, of demonising anybody to his right in the party as a traitor to the true cause, it would be a delicious irony if he went down in history as Ramsay MacCorbyn, the enabler of the most dastardly Tory project since Thatcherism. Brexit has done stranger things.