Despite those victories, Mr. Blair has become a divisive figure within the Labour Party. For some, he is revered as a man of moral courage who made the party electable, brought peace to Northern Ireland and delivered freedom to Iraq. Others revile him as the leader who robbed Labour of its soul and launched an illegal war. (Mr. Corbyn himself believes Mr. Blair could be indicted as a war criminal.)

In truth, Mr. Blair is neither saint nor sinner. His election victories were as much the product of the exhaustion of the Conservative Party after 18 years in power as they were of his political acumen. And he did not so much rob Labour of its soul as recognize that it could no longer be the party that it once was — a party built on a trade union movement whose power had been neutered.

Labour had become a party without a social base or political foundation. Mr. Blair’s solution was to transform it into a technocratic organization built on “triangulation” — a strategy of stealing policies of one’s opponents in order to capture the middle ground, an approach borrowed from Bill Clinton’s 1996 re-election campaign.

The Blair strategy allowed Labour to regain power while the Conservatives were in disarray, but it failed to provide a long-term solution to the party’s need to create a new constituency and social role.

The current leadership battle reflects this dilemma. Mr. Corbyn’s bid to recreate the old Labour Party rooted in the power of the unions suggests a failure to recognize how much Britain has changed. Yet his critics offer no alternative political vision that would engage voters looking for social change.

With the dismantling of the postwar political system has gone, too, the old division between social democracy and conservatism. The new fault line — not just in British politics but throughout Europe — is between an elite, technocratic managerialism, governing through structures that often bypass democratic processes, and a growing mass of people who feel alienated and politically voiceless.

The new divide cuts across old distinctions of right and left. To survive, many of the old parties have embraced the elite, technocratic approach. At the same time, hostility to the established order has led to a rise of populist parties, from the far-right National Front in France to the left-leaning Scottish National Party. (There are signs of a similar process in the United States: Witness the popularity of both Mr. Trump, among Republican voters, and Senator Bernie Sanders, among Democrats.)