As Britain prepares for its most seismic election in decades, one that will decide its relationship to Europe and place in the world, few voters face such an anguished choice as the country’s 300,000 Jews. With anti-Semitism surging in Europe and the United States, the debate over how to vote in Britain’s election has already rattled rabbinical conventions, strained friendships and started a war of letters in the left-wing press, as many Jews wrestle with nerve-racking options.

They overwhelmingly oppose Brexit, the main plank of Prime Minister Boris Johnson’s campaign, dreading an emboldened far right and the splintering of a European project at the root of post-World War II peace.

But they also flinch at handing power to Mr. Corbyn, seen by many as the only person who can stop Brexit, pained as they are by an avalanche of anti-Semitism accusations against the party.

“I feel quite torn,” said Keith Kahn-Harris, a sociologist and writer who attends Alyth. “The issue in the Jewish community at the moment is anti-Semitism is something you can’t hold your nose for, the one thing you can’t overlook, which I understand. But for me, there are multiple things I can’t overlook, and it’s very difficult to know how to balance them.”