LONDON — Boris Johnson hurtled to the top of British politics with an air of charm and disarrayed befuddlement. He slipped into Latin and Greek, changed sides when it suited his ambitions and oozed a mischievous bravado, as when he put his foot on a table at the French president’s palace last week.

But Mr. Johnson’s decision on Wednesday to cut short a session of Parliament revealed another side: the ruthless tactician who took office as prime minister this summer. With Brexit hanging in the balance, Mr. Johnson marshaled all the power of Downing Street to cut out the legs of a wobbly opposition, risking a constitutional crisis to get what he has promised.

Suddenly the man affectionately known as “BoJo” was being rebranded by some opponents a “tin-pot dictator.” And President Trump, known for his own norm-smashing maneuvers, applauded Mr. Johnson, calling him on Twitter “exactly what the U.K. has been looking for.”

Mr. Johnson’s opponents argue that his policies could result in a disastrous no-deal Brexit with the potential to tear apart the United Kingdom, cripple British agriculture and some manufacturing sectors and throw the economy into a recession, while producing shortages of food and medicines.