The result is a confusion that turned the past week into a theater of the absurd after Mr. Corbyn was slammed in the British press for attending a Seder hosted by a far-left, but unmistakably Jewish, group called Jewdas. Jewdas calls itself non-Zionist, and it revels in the history of the radical Jewish diaspora; during the Seder, participants sang Yiddish songs cursing the police (in addition to observing more traditional Passover rituals).

This group was then denounced by the president of the Board of Deputies of British Jews, Jonathan Arkush, as “a source of virulent anti-Semitism.” The board, which has claimed to speak for British Jewry since the 18th century, usually keeps its head down and avoids the headlines. In the 1930s, it held back as other Jewish groups, mostly on the left, led the struggle against a nascent fascist movement on the streets of London. An inglorious role, perhaps, but one that has allowed the Board of Deputies to appear nonpartisan and impartial.

Not this time. Interviewed on TV, Mr. Arkush opined that Jewdas’s members “are not all Jewish,” as if he were in a position to make authoritative pronouncements on the subject.

We are back in the usual arena of communal politics in which notables bandy writs of excommunication as a means of confirming their own authority. What happened to keeping shtum? Is fighting Jewdas really a priority in the struggle against anti-Semitism?

In this business no one comes out well — neither Mr. Corbyn nor his critics. But the lack of perspective and insight that both sides have demonstrated ultimately have less to tell us about anti-Semitism than they do about the diminished state of British politics. Is anti-Semitism a real issue in Britain? Yes. Is it worse for the Labour Party than for others? The evidence suggests not. Is it the most serious manifestation of racial prejudice facing the country? By no means. Muslims have it much worse than Jews, and Eastern European and other immigrants have also been the targets of far-right violence, especially since the referendum to leave the European Union.

In Brexit, Britain faces the most consequential foreign policy decision of the past half-century, one that will transform the country’s position in the world. So far, the government has handled the negotiations like amateurs. Faced with a hostile use of deadly nerve agents on its own territory, the foreign secretary, Boris Johnson, has responded with his characteristic lack of professionalism. If this is how the country’s political elite tackles issues of such gravity, can we be surprised at how Mr. Corbyn, no less parochial in his way than his Conservative opponents, has fumbled his own internal crisis — and how the news media have fanned the flames?