Boris Johnson gives a speech outside 10 Downing Street in London on July 24, the day he was formally appointed British prime minister. (Adrian Dennis/AFP/Getty Images)

BORIS JOHNSON suffered from attenuated democratic legitimacy when he took office as British prime minister last month. Just 92,153 voters, all of them members of the Conservative Party, chose to install him at 10 Downing Street after the resignation of Theresa May. Now, in an attempt to deliver on his promise to take Britain out of the European Union by Oct. 31, “do or die,” Mr. Johnson is further testing the limits of one of the world’s oldest democracies. On Wednesday, he moved to mandate an unusually long parliamentary recess, with the apparent intent of hamstringing opposition to a “no-deal Brexit” — that is, a sharp break with the European Union without agreement on the terms. The result was a political crisis that is sure to further polarize the country.

It’s not obvious that Mr. Johnson’s move was the “coup” or “constitutional outrage” that critics called it. It is normal for new prime ministers to obtain (formally, by order of the queen) a brief parliamentary suspension, which has the effect of clearing all pending legislation. But Mr. Johnson sought five weeks rather than the usual several days; and though he claimed that his intent was to prepare the way for his own governing agenda, virtually all in Westminster concluded that he intended to head off legislation, which opposition parties agreed to pursue Tuesday, meant to block a no-deal Brexit.

If the bill cannot be passed in the few days before the recess, Parliament will not be able to take it up until at least Oct. 14, just a couple of weeks ahead of the deadline. Mr. Johnson says he still hopes to negotiate an exit agreement ahead of an Oct. 17 E.U. summit, allowing time for Parliament to convene and approve it. But he clearly wishes to retain the option to pursue a no-deal exit — even though a majority of the Parliament opposes it and government studies show it could cause far-reaching disruption to the economy.

The best way for opponents to counter Mr. Johnson would be to force new elections; a vote of confidence in his government could come next week. If the Conservative leader won an election between now and Oct. 31, he would have a mandate to pursue Brexit, whatever the cost. Yet Mr. Johnson might seek to dodge that accountability, as well. His aides are telling the British media that he would schedule any election after the Brexit date. He could be unseated before then if Parliament rallied behind an offer by Labour Party leader Jeremy Corbyn to form a transitional government. But many who oppose Mr. Johnson would be equally loath to support Mr. Corbyn, a radical Marxist.

It was not easy to foresee Wednesday what the ultimate consequences of Mr. Johnson’s reckless gambit would be, other than a further weakening of Britain’s polity and its ability to play a meaningful role as a vital U.S. ally. So it was perverse, if predictable, that President Trump chose to cheer on Mr. Johnson, tweeting that he “is exactly what the U.K. has been looking for.” If that were true, Mr. Johnson would have had no qualms about allowing Parliament to debate Brexit — and he would call an election before any exit.