Boris Johnson has made his move. He should find out Friday morning if it pays off.

The safe course for the ambitious former London mayor would have been to back his longtime political ally David Cameron's bid to keep Britain inside the European Union in Thursday's referendum and let the prime minister sink or swim.

Instead, Johnson decided in February to lead the "leave" campaign and use his considerable clout to try to pry Britain out of the EU and, at the same time, push Cameron underwater — many observers believe Cameron's days at 10 Downing Street would be numbered if his "remain" position loses the referendum vote.

That result would — like magic — make Johnson a strong contender for the leadership role.

Longtime Johnson watchers view his perch at the helm of the "leave" campaign through this Downing Street prism, said Tony Travers, a political scientist at the London School of Economics.

"Undoubtedly he has a longer-term ambition, a desire to be leader of his party and therefore prime minister," said Travers, who has tracked Johnson for years. "Now many people at the top of politics have that kind of ambition, (but) few make it that obvious."

Johnson denies he's eyeing N0. 10. He says Cameron should stay on even if Britain votes to leave the EU, but few take this position seriously. Johnson said in February it gave him "heartache" to break with Cameron over Brexit, a British exit from the EU, then jumped into the campaign with customary gusto.

The 52-year-old Johnson has managed to use his disarranged, slightly comical hair as a helmet, shielding him from more serious scrutiny. It lets him come across as an unconventional politician even as he carved a straightforward political path, moving from elite colleges into journalism, then Parliament, then City Hall, and finally back to Parliament and a minor Cabinet position.

He is (almost) always willing to play the buffoon, not minding when he's photographed stranded on a zip line looking ridiculous and happy to speak extremely elementary Greek to Greek constituents. Johnson emphasizes his American connections with American visitors (he was born in New York City's posh Upper East Side) and talks comfortably about his Turkish great-grandfather with Muslims, sometimes pointing out that his ancestor studied the Quran.

"His detractors and critics often say he is not a serious person, that his approach to politics is humorous and freewheeling and perhaps not as thought through as other people," said Travers. "I'm not sure that's true. I think deep down he is a serious person, with serious objectives."

Johnson is not alone on the "leave" campaign stage. UK Independence Party chief Nigel Farage has played a prominent role, despite recently coming under criticism for a provocative poster, and Justice Secretary Michael Gove has been a focal point as well.

They lack Johnson's flair for publicity, however. Johnson has become the face of the "leave" campaign and, perhaps inevitably, the referendum campaign is seen by some as a Cameron versus Johnson race, with Downing Street as the prize.

Johnson's decision to break with Cameron over Brexit marks the first public split between the longtime friends and sometimes rivals. They have known each since university days — the two were famously photographed in formal wear during their days as members of the exclusive Bullingdon Club at Oxford.

Throughout his career, Johnson has managed to surmount the sort of gaffes that have brought other politicians down. Some believe this Teflon resilience would allow him to keep Downing Street in his long-term sights even if Britain votes to stick with Europe.

In his newspaper days, Johnson called Africans "piccaninnies" and referred to people from Papua New Guinea as cannibals. As a Member of Parliament, he offended an entire British city when he complained that people from Liverpool were wallowing in "victim status" after a Liverpudlian was taken hostage and slain in Iraq.

He apologized — a lot — and seemed to be forgiven, a lot. But not everyone is impressed.

Joanna Wright, a driver on London's Underground train network known as the Tube, said she's lost respect for Johnson, in part because of his treatment of Tube workers when he sought to make it a 24-hour service.

"He doesn't have much respect for us either," she said. "The most recent example would be announcing the night Tube when none of us, who are working at the Underground, had been consulted. I think he likes to put out policies without actually consulting with people who are going to be doing the things he is asking."

She said she would be "very worried" if Johnson becomes Britain's leader.

"People get off on the way he looks and his personality, but I think his policies are quite selfish," said Wright, 49. "I don't think he is a politician for normal working people. I don't believe in him as a serious politician."

Others hope a result in favor of Brexit would give Johnson the push he needs to take over at No. 10 — they plan to vote that way with that prospect in mind.

Mike Jaselsky, who owned a courier service before retiring, said he wants Johnson to become prime minister but thinks he will be blocked unless the "leave" campaign wins.

"He is a very clever man," said Jaselsky, 68. "He acts like a buffoon, but he is not...For me he is the main man."


Jonathan Shenfield and Leonora Beck contributed.