But Britain has lagged somewhat in instituting these reforms. Where it has tried them, for instance in allowing a primary-style open election of party leaders, populist forces have risen. Where Britain has not yet followed, its institutions and norms have held.

Rank-and-file British lawmakers, for example, still win their party’s nomination by being selected rather than running in a primary. So while Mr. Johnson can become party leader by appealing to its most fervently ideological members, his own coalition can more easily ignore those voters, as they have done in halting their leader’s hard-line Brexit strategy.

When Hardball Fails

Legal scholars have a phrase for moves like Mr. Johnson’s attempt to suspend Parliament: constitutional hardball.

He was technically within the rules of British democracy, which allow the prime minister to “prorogue” Parliament, suspending it with the monarch’s assent. But he was exploiting those rules for political advantage, excluding Parliament for much of a Brexit debate that he was likely to lose. And he did so at the expense of unwritten norms, which favor involving Parliament in a decision of that magnitude.

Constitutional hardball of this sort has been a hallmark of the populist era. It is encouraged by political polarization, which leads supporters of one party to see their opponents as so dangerous that stopping them is more important than safeguarding democratic norms. It is favored by populist leaders who see strong-handed rule and smashing the system as upsides in their own right.

But it can be dangerous. It forces opposition parties to either fight back in kind, risking a tit-for-tat cycle that has led democracies to erode or collapse outright, or to show restraint at the cost of accepting a potentially permanent disadvantage.