Rabbis used to joke that when young Jews became a bar or bat mitzvah, tradition held that they be given lifelong membership in the Labour Party. But no longer. Many Jews now feel sharply at odds with Labour, and politically homeless in Britain.

A number of Labour lawmakers have quit the party over the issue since February. The Jewish Labour Movement, a 100-year-old socialist group, decided not to campaign nationally for Labour for the first time in its history. And the Jewish Chronicle, Britain’s most influential Jewish newspaper, ran a front-page editorial beseeching non-Jews to heed the paper’s fears of a Corbyn government.

The dread felt by British Jews reflects wider alarm over a rise in anti-Semitism on the hard left and the hard right in Europe and the United States. Britain’s Conservative Party, pulling a page from Republicans in the United States, has tried to capitalize, courting Jews angered by the anti-Semitism scandal within Labour.

But some British Jews are wary of seeing the cause weaponized in the heat of an election. Distrustful of Mr. Johnson’s own record of racist and Islamophobic remarks, and fearful of far-right violence against Jews in the United States, they are trying to persuade people to consider how a Johnson government could hurt not only Jews but other vulnerable groups.