Long before Britons lined up for Leave or Remain, Americans divided over the rise of Mr. Trump, who criticized his rivals with a venom seldom seen in domestic politics, who expressed views on race seldom shared in public and who defied the establishment of a political party that itself was a symbol of the national establishment. He belittled the Bushes, the royal family of the Republican Party, even as he ridiculed the party leaders who control the legislative branch and who would, in a Trump administration, hold in their hands the destiny of the Trump agenda.

And yet there is little question that Mr. Trump will use the British vote as a bludgeon against Ms. Clinton, who herself has just completed a battle against a populist insurgency in her own party. The campaign of Sen. Bernie Sanders of Vermont was based on the same anti-establishment forces that propelled Mr. Trump to victory, and the hurricane-force winds of the Sanders campaign, which blew away Ms. Clinton’s aura of invincibility, pushed the former New York senator to adjust her views on trade agreements and to adopt an anti-business profile at odds with her recent history and perhaps her inclinations.

Campaigning as a populist is easy; governing as one is far more difficult. Mr. Trump has mastered the former. Ms. Clinton will argue that the latter is incompatible with American tradition. Thus the great populist moment is itself about to be transformed into a truly significant November confrontation of ideas and institutions.