The crisis deepened with news on Thursday of the resignation of Christine Shawcroft, the chair of Labour’s internal disputes panel, after the leak of an email suggesting that she had lent support to a council candidate suspended after accusations of anti-Semitism.

Coming after criticism of Mr. Corbyn’s reluctance to blame Moscow for the poisoning attack on a Russian former spy and his daughter in Salisbury, England, the issue has recharged a bitter argument about his leadership. That, in turn, has led Mr. Corbyn’s supporters to complain of a politically motivated plot to destabilize his position because of his left-wing politics.

But there is little doubt that the atmosphere in the Labour Party has changed. A veteran of left-wing struggles, Mr. Corbyn learned his politics fighting apartheid in South Africa and campaigning against what he saw as Western militarism and neo-imperialism. While many of those battles have been won, the plight of the Palestinians remains a sore point on the left, one deepened by the hard-line policies of the Israeli prime minister, Benjamin Netanyahu.

At the same time, the Labour Party’s membership has grown bigger and more diverse, while Momentum, a campaign that promotes Mr. Corbyn’s radical brand of politics, has emerged as a powerful force, tilting the political balance further to the left.

This is not the first time that Mr. Corbyn’s party has faced allegations of anti-Semitism. In 2016, one lawmaker, Naz Shah, was temporarily suspended for sharing a graphic showing an outline of Israel superimposed on a map of the United States, suggesting the relocation of Israel to North America.

When the former mayor of London, Ken Livingstone — a longstanding ally of Mr. Corbyn — tried to defend Ms. Shah, he instead poured gasoline on the flames by arguing that, in 1932, Adolf Hitler “was supporting Zionism before he went mad and ended up killing six million Jews.”