There is little question that Labour’s opponents — and internal critics of the party’s left-wing leader, Jeremy Corbyn — have exploited the issue for factional gain. Nor is it hard to see the hypocrisy. Boris Johnson, the current mayor of London and a leading figure in the Conservative Party, said that Labour had been infected by the “virus of anti-Semitism.” A week earlier, he had exposed his own racial sensitivity, dismissing President Obama’s support for Britain’s membership of the European Union as the animus of a “part-Kenyan president” with an “ancestral dislike of the British Empire.”

Yet neither the cynicism nor the hypocrisy should distract us from the problem of anti-Semitism — not just in the Labour Party, but on the political left more generally. It is not that the left is packed with anti-Semites; rather, too many among them have been willing to accommodate bigotry.

This acquiescence is rooted in the changing character of the left in recent years. Anti-Semitism used to be a problem primarily of the right. It wasn’t that the left had a totally clean bill of health — there is a history of left-wing anti-Semitism — but its firm foundation of universal values and egalitarian principles established a proud tradition of fighting bigotry against Jews.

In recent decades, however, much of the left has retreated from these commitments. Where before radicals challenging inequality and oppression did so in the name of universal rights, many now stress multiculturalism, celebrating a world divided into distinct cultures, each with its own ideas, beliefs and values. Such “identity politics” turns on its head the dictum of the Rev. Dr. Martin Luther King Jr. that one should judge people “not by the color of their skin but by the content of their character.” Once identity becomes the primary feature of political life, then people are judged as much by the group to which they belong as by their character or principles.

Identity politics has made it easier to hold all Jews responsible for the actions of the State of Israel and to go after Jews simply for being Jews. As the distinction between criticizing ideas and fingering a group has eroded, there has been a slippage from anti-Zionist activism into outright anti-Semitism. Many who support the Palestinians now seem genuinely unable to distinguish between criticizing the policies of the Israeli government and sowing hatred against a people.