American politics are radicalizing. The damage will last generations.

In the world of progressive politics, all eyes are turned to Britain. Jeremy Corbyn, leader of the Labour Party, represents progressivism in its most uncompromising form. He and his party are proposing massive increases in social spending, tax hikes on businesses and the wealthy, rent control in major cities, a ban on fracking, a boost in the minimum wage and the re-nationalization of railroads and water companies. Corbyn himself has advocated unilateral disarmament, has urged the United Kingdom to leave NATO and has seldom found a socialist revolutionary he didn’t admire (including Hugo Chávez).

And according to a recent YouGov-Times poll, Corbyn’s Labour Party is one point behind the Conservatives in voting intention.

There is no immediate election on the horizon in the United Kingdom. And the disturbing ties between British leftism and anti-Semitism are emerging as a serious scandal. But there is little doubt that Corbyn’s forces have consolidated their hold on the Labour Party, that the party did better than expected in the 2017 election and that Corbyn is no longer unthinkable as a future prime minister.

Whatever else Corbyn’s ascendance might mean, it is the death of Blairism — former prime minister Tony Blair’s attempt to define a center-left alternative to the Labour Party’s hard left. No more political trimming and tacking. Corbyn supporters regard themselves as part of a people-powered social movement — dedicated to economic equality and environmental protection, opposed to militarism and in revolt against a compromised establishment.

There is no exact political equivalent to Corbyn himself in the United States, at least outside the faculty lounge. But a similar spirit could be seen in Bernie Sanders’s 2016 presidential campaign — the romance of ideological purity. Sanders’s supporters were as opposed to (Bill) Clintonism as Corbyn’s are to Blairism, and for the same reasons. Many on the left have lost patience with centrism. They feel part of a progressive wave, a movement. They see no need to compromise, just to organize.

In the United States, this tendency on the left is reinforced by President Trump’s consolidation of power in the Republican Party. Trump’s extremism — his combination of plutocracy, misogyny and nativism — has encouraged ideological ambition in his opponents. His vulnerability is taken not as an opportunity to build a broad political coalition against Trumpism — but as a chance to win without compromise. A chance to bury conservatism itself.

Compared with Britain, this is a big and diverse country. So this trend on the left is not found everywhere equally. But consider recent events in California. In February, the state Democratic Party declined to endorse Sen. Dianne Feinstein for reelection. Though a liberal by almost any standard, Feinstein was not liberal enough for delegates at the party convention. Her challenger, state Sen. Kevin de León, attacked her support for school vouchers, for anti-terrorist surveillance, for “a criminal justice system propped up by institutional racism” and for the Iraq and Afghan wars. “The days of Democrats biding our time, biding our talk, are over,” de León told cheering delegates. “Leadership comes from human audacity, not from congressional seniority. . . . We demand passion, not patience.”

Some progressives talk of California — with its political argument between left and lefter — as a model for the nation. A recent (and much tweeted) article by Peter Leyden and Ruy Teixeira concludes that bipartisanship is dead because the GOP is no longer a functioning partner. Describing our politics as a new civil war, they argue: “At some point, one side or the other must win — and win big. . . . Now the entire Republican Party, and the entire conservative movement that has controlled it for the past four decades, is fully positioned for the final takedown that will cast them out for a long period of time in the political wilderness. They deserve it.”

In the GOP, fanaticism seems to have all the passion and energy. On the left, the same is increasingly true. But there are problems when politics ceases to be the realm of partial agreement and becomes a conflict of social movements. The virtues essential to self-government — civility, compromise and moderation of temperament — are devalued. The incremental reforms necessary to solve public problems become impossible. Opponents are dehumanized and viewed as enemies. The cruel and intemperate come to dominate our political life.

Simply put: If the response to Trump is a general radicalization of American politics, the damage will last generations. Somehow, in the midst of so much fanaticism, moderation must find a passion of its own.

Read more from Michael Gerson’s archive, follow him on Twitter or subscribe to his updates on Facebook .