Photographer: Chris Ratcliffe/Bloomberg

The Fight to Keep Britain in the EU Just Got a Lot Harder

Boris throws a wrench in Cameron’s plans to avoid a Brexit.

Not quite a year ago, British Prime Minister David Cameron achieved the impossible and won a majority in Parliament. It’s a small majority, but the internal battles of the Labour opposition have left the impression that Cameron is dominant in British politics. So it’s surprising that he might have to resign in June. If he does, he’ll have been forced out not by a rival or by a scandal, but by the misfiring of a three-year-old gambit of his own making.

Europe has been the issue that’s caused the Tories the most difficulty ever since a fight over the extent of British integration into the European Union helped bring about the fall of Margaret Thatcher in 1990. Successive leaders were either chosen for their purity on the issue—less Europe, which was Thatcher’s stance—or undermined for their lack of it. By 2005, when Cameron took the job, it was impossible to be elected leader without making at least some noise about distancing Britain from the EU.

Amid this ideological debate, the public turned its back on the Tories. It wasn’t, Cameron argued in a 2006 speech, a coincidence. “While parents worried about child care, getting the kids to school, balancing work and family life, we were banging on about Europe,” he told his party. For the next four years, as party leader he focused the Conservatives on domestic issues, with enough success that he became prime minister in 2010, ending 13 years of Labour rule.

Cameron had moved on, but his party hadn’t. For years, a group of anti-EU activists dominated the Conservatives’ internal process of picking candidates for seats in Parliament. While Cameron threw the occasional Brussels-bashing bone to euroskeptic MPs, it wasn’t enough: They demanded more restrictions on EU powers.

Hence the prime minister’s 2013 gamble. In an effort to satisfy his own side, he promised a referendum on leaving the EU by the end of 2017. Cameron’s hope was that this pledge would get him through the 2015 election without more rebellions or any defections by members of Parliament to the anti-EU U.K. Independence Party. He failed on both counts.

His further calculation was that, if he was still in office after the 2015 election, he’d be able to secure sufficient concessions from his EU partners that a risk-averse Britain would vote to keep things as they are. “While many British are euroskeptics, they also appear deeply anxious over the perceived risks of Brexit,” says Matthew Goodwin, a professor of politics at the University of Kent, using the shorthand term for Britain’s possible exit.

A majority vote for staying in the EU still seems likely. Cameron may have misread the situation in his party, though. The Tory MPs, despite their tough talk on Europe, were expected to fall in behind their prime minister and campaign to stay in the EU. But since he announced on Feb. 20 that a referendum would come on June 23, around half of them have said they’ll campaign against him. Worse, among the rebels are one of his oldest political friends, Justice Secretary Michael Gove, and one of Britain’s most popular politicians, London’s Tory mayor, Boris Johnson. Johnson’s announcement of his support for Brexit triggered a drop in the pound to its lowest level in seven years, as traders sized up the possibility that Cameron might lose. “Cameron’s not exactly renowned for having his finger on the pulse of his parliamentary party,” says Tim Bale, a professor of politics at Queen Mary University of London.

The mayor, universally known as Boris, is a megastar who operates in a different space from mere prime ministers. With his shaggy hair, tendency to lapse into Latin, and carefully bumbling self-deprecating manner, he’s managed to escape scandals, both political and sexual, that would destroy anyone else. Having said as a child that he wanted to be “World King,” the ex-reporter has moderated his ambitions. If anyone was uncertain about them, his latest best-selling book is a study of another maverick former journalist who went on to lead his country, Winston Churchill.

It remains to be seen if Johnson can win voters to the Brexit camp. “Cameron will warn that leaving would be a leap in the dark, and the trouble is that Boris quite likes leaps in the dark and taking risks,” says Andrew Gimson, Johnson’s biographer. “That doesn’t necessarily make people feel safe.” Few Londoners know what their mayor does, and it’s been Johnson’s good fortune to be in charge at a time when the city has enjoyed a run of successes, including the 2012 Summer Olympics. Cameron remains the serious leader, arguing that he’s motivated solely by what’s in the nation’s interest—unlike, goes the unspoken suggestion, the ambitious Johnson.

Polling evidence on how the vote will go is unclear. Some show the two sides neck and neck; others show a clear majority of Britons voting to stay in the EU. All the polls have shown a move toward the “leave” side in recent weeks. The differences among the polls may reflect the public’s lack of engagement with the question: The EU is something about which few Britons care deeply.

If Britain votes to leave, it’s hard to see Cameron staying on as prime minister. Even if the nation votes to stay, the first days of the campaign have shown how deeply the issue of Europe continues to affect the Conservative Party. It suggests that opposition to membership continues to be a requirement for a Tory leader. That’s a big problem for Cameron’s favored successor, Chancellor of the Exchequer George Osborne, who’s campaigning at his leader’s side. It’s clearly an opportunity for Johnson.

The bottom line: The Conservatives are set for a leadership fight after the Tory mayor of London breaks ranks with the prime minister.

'Brexit': What Investors Need to Know