It may be that Britain’s political slide into Boris Johnson’s populism was inevitable in the struggle over Brexit. But his initial onslaughts against the “mother of Parliaments” have been rebuffed with laudable resolve.

The Brexit referendum in 2016 elevated the question of membership in the European Union to a faith-based schism that touched every British nerve from Ireland to lost empire, evenly sundering the British electorate and confounding Britain’s tradition-bound political system.

Somewhat like other populists who took advantage of discontent and confusion to win power, Mr. Johnson emerged as prime minister from a deadlocked Parliament in July by proclaiming himself the true voice of “the people,” assailing as traitors all those who disagreed with his eagerness to break with the European Union even without a negotiated deal to smooth the exit. With an Oct. 31 deadline looming for a no-deal Brexit, he labeled legislation to bar that possibility a “surrender bill” and accused legislators who wanted to remain in the bloc of “collaboration” with Brussels.

That’s the stuff of modern-day populism, and, as Max Fisher wrote in The Times’s The Interpreter column, Mr. Johnson promptly followed its well-thumbed playbook by manipulating the rules, conventions and procedures of British politics to try to consolidate power over Parliament and his own Conservative Party, undermining both in the process. He tried to exploit a political rule to curtail parliamentary debate, purged his party of dissenters and started maneuvering to hold early elections.