When Boris Johnson’s Conservative Party won a landslide election victory in 2019, he loomed as a colossus over British politics, the man who had redrawn the country’s political map with a vow to “get Brexit done.”

Mr. Johnson appeared to be assured of five years in power with an 80-seat majority in Parliament, the largest amassed by a Conservative leader since Margaret Thatcher in 1987. Some analysts predicted that Mr. Johnson, the most consistent vote-getter in British politics, would spend a comfortable decade in 10 Downing Street.

Mr. Johnson’s future was hanging by a thread on Monday, less than three years after that triumph. Rebels in his party have called for a no-confidence vote that could cost him his job; even if he wins and clings to his position, it could cripple him as an effective and credible leader. On Monday night, he will face a vote from his own party.

It is one of the most perplexing turns of events in modern British political history.

Mr. Johnson’s standing eroded to some extent because of the same perplexing mix of strengths and flaws that propelled his rise: rare political intuition offset by breathtaking personal recklessness; a sense of history that was not matched by a corresponding sense of how he should conduct himself as a leader; and uncanny people skills tainted by a transactional style that earned him few allies and left him isolated at critical moments.

Analysts believe that Mr. Johnson’s vulnerability to setbacks stems from this last quality. With no underlying ideology other than Brexit and no network of political friends, the prime minister lost the support of his party’s lawmakers when it became clear that he would not win the next election.

Mr. Johnson, after all, is the politician who decided to back Brexit after writing two columns the night before, one arguing for leaving the European Union and the other arguing against it. He was elected in 2019 on the promise of “getting Brexit done,” but after accomplishing that goal within months of taking office, he frequently appeared to be a prime minister without a plan.

Mr. Johnson reacted late to the virus’s looming threat, imposing a lockdown on the country a week after neighboring countries. According to critics, the delay made the first wave of the pandemic worse in the UK than elsewhere. Mr. Johnson contracted Covid in April 2020, while the virus was circulating in Downing Street, ended up in an intensive care unit, and nearly died.

Mr. Johnson, on the other hand, advocated for Britain to be a pioneer in the development of a vaccine. When Oxford University and AstraZeneca created one, he released it faster than nearly every other major country. He also made a fateful decision, which other leaders followed, to reopen society after a significant percentage of the population had been vaccinated. He stated that Britons must learn to live with Covid.

The seeds of Mr. Johnson’s current problems were sown during the pandemic’s darkest days. While the rest of the country was subjected to suffocating lockdowns, the prime minister and his top aides were attending Downing Street social gatherings that violated their own lockdown restrictions.

When the first reports of illegal parties surfaced late last November, Mr. Johnson issued a blanket denial that any laws had been broken. A subsequent police investigation revealed that this was not the case: Mr. Johnson was fined for violating the rules by attending his own birthday party.

Mr. Johnson’s supporters argue that “Partygate,” as the London tabloids dubbed it, is a minor distraction at a time when Europe is facing its first major land war since World War II. The prime minister quickly established himself as Ukraine’s staunchest defender, sending powerful weapons to its army and calling his new friend, Ukrainian President Volodymyr Zelensky, on a regular basis.

The taint of moral hypocrisy corroded the prime minister’s public popularity. When he and his wife Carrie Johnson climbed the steps to St. Paul’s Cathedral on Friday for a thanksgiving service honoring Queen Elizabeth II’s 70 years on the throne, the crowd booed him. It was a sign.