The implosion in Britain of the hard left “Your Party” concocted by Jeremy Corbyn and Zarah Sultan marks another milestone in the long-running tradition of leftist self-sabotage. The former leader of Labor has now apologized for the “fraught days in the last week” and acknowledged that “we haven’t covered ourselves in glory.” The rift appears to stem from an effort by Ms. Sultan to unilaterally launch a membership portal under Mr. Corbyn’s nose.

That membership scheme invited Britons to sign up to the new party for £5 a month or £55 a year. Mr. Corbyn, who once ran for prime minister, called this plan “unauthorized.” Your Party later posted on X that “For those who joined under the previous system last week, please be assured your data and membership is secure … There is nothing more for you to do at this stage.” Ms. Sultana has railed at “the baseless and unsubstantiated allegations against me.”

Ms. Sultana alleges that she has been “sidelined” by a “sexist boys club.” She adds that she has been “treated appallingly and excluded completely.” One poor soul interviewed by Sky News, identified as “Sam T,” said he “might as well go and give £5 to someone on the street,” Another, a Glaswegian named Layla C, ventured that “the right-wing will love this and it’s not looking good.” Mr. Corbyn explains that he aims only to “steward the founding of the party.”

A leading leftist, Bhaskar Sunkara, writes in the Nation that Your Party has devolved “into bitter factional warfare” at an inopportune time — when the Reform party of the Brexiteer Nigel Farage “has enjoyed a commanding polling lead for months and is the favorite to form the next government.” Meanwhile, Sir Keir Starmer’s Labor government is adrift, unpopular at home and squandering credibility abroad with its recognition of a Palestinian state. 

More than schadenfreude is at play in rooting against Your Party’s viability, notwithstanding its claim that some 800,000 people have signed up. Helen Lewis in the Atlantic explains that “Your Party exists because of the Israel-Palestine conflict.” Ms. Lewis reckons that “the British left … has two shibboleths that potential leaders are required to utter. The first is that Israel is committing a genocide. The second is that ‘trans women are women.’”

Mr. Corbyn’s commitments on this head are no secret. He led Labor into a dead end of antisemitism so bad that Mr. Starmer, himself no Jabotinsky, eventually expelled the Islington North MP from the party. Mr. Corbyn has broken bread with Hamas leaders, paid homage to the graves of terrorists who perpetrated the Munich Olympic massacres. He also accused “British Zionists” of possessing “no sense of English irony.”

The Onion once ran the headline that declared “Left-Wing Group Too Disorganized for FBI Agents To Infiltrate.” The stumbles of Your Party, though, does not mean that the threat of leftist politics has been neutralized. On the contrary. That such a cockamamie cause could rally the support of so many is a baleful development in a country where antisemitism is spiking. Mr. Corbyn and his ilk may be down, but they are far from out.