Read the original article at the Wall Street Journal

Ex-London mayor was de facto leader of campaign against EU referendum

Boris Johnson, the former London mayor who campaigned for Britain to exit the European Union, scored a major victory and is now a leading candidate to succeed Prime Minister David Cameron, who led the losing camp.

Just a year ago, Mr. Johnson was a crucial weapon for the Conservatives in the general election that gave the party full control of the U.K. government and Mr. Cameron a second term as prime minister.

But in February, Mr. Johnson made a dramatic pivot against his party and prime minister by joining the Leave effort. The ensuing campaign served as a showcase for Mr. Johnson’s quick-witted and often unsparing brand of politics. He trained his verbal guns on Mr. Cameron, a longtime political ally and friend, launching sharp-tongued broadsides that belittled the arguments Mr. Cameron made for staying in the EU.

The campaign also gave Mr. Johnson, 52 years old, exposure to a much wider U.K. audience than he had as London mayor.

On the heels of Thursday’s vote, the question will be whether Mr. Johnson now gets to No. 10 Downing Street.

Mr. Cameron has said he planned to stay in his job whatever the result of the referendum. He said his job would be to implement the will of the people.

But having lost the referendum, Mr. Cameron’s position could become untenable, and he could resign or face a leadership challenge.

He has spent months campaigning fiercely against many lawmakers in his own party who favored leaving the bloc, including Mr. Johnson, who might be offered a cabinet job.

Mr. Johnson has said he wants the party to unite behind Mr. Cameron after the referendum. He has said he would be interested in being prime minister if the post were to become available, but has declined to comment on his ambitions during the referendum campaign.

In an interview this month on the campaign trail, Mr. Johnson dismissed a question about whether he wanted the top job by saying there was too much attention on “entirely irrelevant speculation.”

Mr. Johnson announced that he would be campaigning for so-called Brexit less than two days after the prime minister had trumpeted his deal with other EU leaders as securing significant concessions for Britain.

He said he made his decision “after a huge amount of heartache” because the last thing he wanted was to go against Mr. Cameron. But he added that he wanted “a better deal for the people of this country, to save them money and to take back control.”

The move prompted some—including the prime minister—to suggest the former mayor was motivated by political ambition. Speculation about who will next lead the Conservatives has ebbed and flowed since Mr. Cameron said early last year that he wouldn’t seek a third term.

Winning two terms as mayor established Mr. Johnson’s vote-getting pedigree. As mayor, he boosted his international credentials in a series of overseas trade missions, and he became one of the most recognizable British politicians after fronting the 2012 London Olympics.

The referendum campaign has allowed him to bring his characteristic humor to an even wider domestic audience. He claimed the EU dictated the shape of bananas and likened the bloc to underwear that had “become too tight in some places—far too tight, far too constrictive—and dangerously loose in other places.”

But his prominent role also made him a lightning rod for pro-EU Conservatives, with Mr. Cameron accusing him indirectly of peddling untruths.

In one of the sharpest attacks, former Prime Minister John Major said Mr. Johnson was an “engaging and charming court jester” and warned if he continued to divide Conservatives he would find that if he ever became prime minister, he wouldn’t have the loyalty of the party.

One senior Conservative lawmaker said in advance of the referendum that divisions in the party over Europe would heal more quickly if there were a vote to leave. “That’s not to say that is going to be an easy process,” the lawmaker said.

Ben Harris-Quinney, chairman of the Bow Group, the U.K.’s oldest conservative think tank, said Mr. Johnson will have a limited time to make the most of any referendum dividend.

“For Boris the sooner a leadership challenge comes the better,” he said. “If Cameron stays on longer than 12 months there is a real danger that other candidates will emerge and will have the time to establish their name and their position with the membership base.”