London's mayor Boris Johnson threw his considerable influence behind the push for a British exit from the European Union on Sunday, Feb. 21. (Reuters)

Mayor Boris Johnson, one of the most charismatic figures in U.K. politics, threw his considerable influence behind the push for a British exit from the European Union on Sunday, instantly galvanizing the campaign and potentially tipping the country toward a vote to leave in a June referendum.

The decision to defy Prime Minister David Cameron, a fellow Conservative, represents a personal gamble for the ever-irreverent politician and a major coup for the “leave” camp, which has been riven by bickering factions and has lacked a high-profile, broadly palatable advocate.

Although Johnson had long been coy about his intentions, the conventional political wisdom in Britain only days ago was that the mayor would take the safe bet and support staying in. But Johnson exploded that belief on Sunday afternoon when he walked out the front door of his gray-brick rowhouse in north London and casually announced to waiting reporters that he would call for Brits to cut ties with the E.U.

“I don’t think there is anything else I can do,” he said, speaking, as is custom, without notes and in tones that alternated between soaring oratory and shambolic muttering. “I will be advocating Vote Leave — or whatever the team is called; I understand there are many of them — because I want a better deal for the people of this country, to save them money and take back control.”

The mop-haired mayor’s announcement adds a new layer of meaning to a referendum already laden with consequence. When Britons vote June 23, the ballot will ask simply whether the country should stay in the E.U. or go.

Prime Minister David Cameron and his E.U. counterparts struck an agreement following marathon negotiations in Brussels. Here's what that means for the country and the possibility of an E.U. without Britain. (Jason Aldag,Adam Taylor/The Washington Post)

But also at stake will be the future of the 28-member bloc, the viability of the United Kingdom itself and Britain’s standing as Washington’s closest and most important ally.

Now, the keys to 10 Downing Street could be on the line, as well.

The 51-year-old Johnson, who was born in New York but said last year he was giving up his U.S. passport to avoid taxes, denied Sunday that his E.U. call had anything to do with his at times barely concealed ambitions to secure the top job in British politics. In answers to reporters’ questions, he insisted that Cameron “has got to stay” as prime minister even if the country chooses “Brexit.”

But if Johnson ends up leading the country out of Europe, persuading a majority of British voters to ignore the prime minister’s passionate call for the country to stay in, there will almost certainly be intense pressure on Cameron to resign. And Johnson, having seized the populist moment, would be uniquely positioned to take his place.

Cameron once named Johnson as one of three possible successors after the prime minister steps down, as planned, before the end of his second term in 2020; the other two have both backed the “in” campaign. After Johnson’s announcement Sunday, critics assailed him for political opportunism, but bookmakers also made him the favorite to become the next prime minister.

Johnson was the last major British politician to reveal his stance on the E.U. vote, with all of the others declaring their intentions long ago or on Saturday — when Cameron officially announced the referendum date. In keeping with Johnson’s reputation for high-profile stunts and ego-enhancing maneuvers, he kept both sides guessing throughout Sunday — and forced both to plea for his backing.

London Mayor Boris Johnson, left, and British Prime Minister David Cameron in 2012. (Will Oliver/AFP/Getty Images)

Cameron took to the BBC on Sunday morning to ask that Johnson carefully consider the impact of his choice.

“I would say to Boris what I say to everybody else, which is that we will be safer, we will be stronger, we will be better off inside the E.U.” Cameron told host Andrew Marr.

But perhaps anticipating that the mayor had already locked in his decision to side with the “leave” camp, Cameron also took some digs at Johnson, musing about whether the mayor wanted to be “linking arms” with Nigel Farage and George Galloway — prominent E.U. critics who inhabit, respectively, the far-right and far-left extremes of British politics.

Until Johnson’s announcement, only relatively marginal political figures had supported a Brexit. The choice to cut ties with Europe is more popular at the grass-roots of British politics than it is among elected officials. Recent opinion polls show that the “in” and “out” camps are effectively tied.

That’s a striking shift from the landscape only months ago, when a clear majority of Brits seemed to favor staying in the union that has defined the postwar political and economic order on the continent.

But Europe’s struggles with the refugee crisis, its persistent economic malaise and its long-standing pattern of dysfunction have all contributed to rising dissatisfaction among Brits — and a growing desire to sever a decades-long relationship.

The consequences of such a choice would be felt globally. Britain has long been an ambivalent member of the E.U., deeply skeptical of political integration and refusing to go along with major initiatives such as the common currency and the passport-free Schengen zone.

But it is also one of the union’s cornerstone members, with a seat at the U.N. Security Council, a nuclear arsenal and the world’s fifth largest economy. The country’s departure could cause the E.U. to crumble at a time when it is already inundated with crises and facing rising Euroskepticism continent-wide.

Britain’s E.U. partners have all pleaded for the country to stay, as has the United States. President Obama has said that the E.U. and Britain are better off with the country inside the bloc. U.S. officials have suggested that British influence would be vastly diminished if the country chooses to get out, threatening the “special relationship” between the two nations.

The United Kingdom itself could be vulnerable to a breakup if the country votes to leave the union: Scotland is considerably more pro-E.U. than England, and Scottish leaders have said they will revive a call for independence should Britain opt to leave. A referendum on independence two years ago went down to a relatively narrow defeat.

Cameron has staked his legacy to settling both of Britain’s great existential questions — one country or two, part of Europe or not. In both cases, he has favored the status quo.

But now Johnson, whom critics regard as a craven opportunist, could disrupt those plans.

A recent poll by Ipsos Mori sheds light on why his endorsement of the “out” camp matters: A third of British voters said they would take Johnson’s views into account when making up their minds on the E.U. (And 44 percent said the same about Cameron.)

A gifted communicator, Johnson has cultivated a bumbling, gaffe-prone public persona that has somehow worked to his advantage, making him relatable to voters outside the London political bubble and diverting attention from his thoroughly upper-crust English background.

Less charming politicians might find themselves in trouble if they got stuck dangling from a zip-line during the Olympics or had flattened a 10-year-old boy during a rugby match in Japan. But Johnson has a unique way of disentangling himself from mishaps both large and small.

His eight-year tenure as mayor of London has received mixed reviews — he has often seemed disengaged and distracted by his roles as a Winston Churchill biographer, newspaper columnist and television personality — but he is a colorful character who stands out from other identikit politicians.

Still, hitching his wagon to the “leave” camp represents a big risk. If Britain votes to stay in the E.U., Cameron could revoke his offer of a top cabinet job when Johnson steps down as mayor in May.

Johnson is not the only defector. On Saturday, six of Cameron’s senior ministers came out in favor of Brexit, including his close friend, Justice Secretary Michael Gove.

But unlike Gove, a Euroskeptic of long-standing, Johnson has a complicated history of views on Europe that have at times seemed to sway with the political winds.

Tim Farron, leader of the pro-E.U. Liberal Democrats, took aim at that inconsistency on Sunday, saying, “Boris has had more positions on Europe than the Kama Sutra.”

Johnson himself insisted that his desire to see Britain leave the E.U. was deeply rooted and sincere.

The stance, Johnson said, may lead some to depict him as “a crazy crank. But I happen to think I am right.”