From his disheveled blond mop to his penchant for Greek mythology, to his off-the-wall comments about how he's more likely to be reincarnated as an olive than become prime minister, Boris Johnson, the London mayor and member of Parliament for the Conservative Party, has a reputation as a wild card on the British political scene. Despite his capricious persona, Johnson still succeeded in surprising many in Parliament on Sunday when he voiced support for the “leave” vote in an upcoming referendum on whether the U.K. will remain in the European Union, effectively opposing the position of Conservative Party leader, Prime Minister David Cameron.

Johnson’s support of a so-called Brexit — ahead of a referendum slated for June 23 — breaks with Cameron’s wishes to stay in the EU after negotiating a new deal with European Union leaders. The London mayor has supported Britain’s membership in the past, particularly given how entrenched the capital is in European and global trading, and some political experts said his decision reflected his desire to become prime minister more than any fervent euroskepticism.

Boris Johnson London Mayor Boris Johnson pictured at Downing Street in May 2015. Photo: Getty Images

“It stacks up in terms of his own personal ambition,” said Charles Lichfield, an analyst for the political risk consultancy Eurasia. “He’s a clever man and he knows that he’s making very spurious claims of how easy it would be for Britain outside the EU.”

The debate over a Brexit has heightened in the past month, with issues of the nation’s sovereignty, its border security concerning migration and its economic future taking center stage. Activists in favor of leaving the EU have argued that existing outside of the European governing body would allow the U.K. better control over immigration as well as freedom from strict EU trading regulations. Anti-Brexit campaigners say EU membership has been a critical part of maintaining Britain’s safety and economic prowess, arguing that uncertainty and economic risk would only increase as a nonmember.

Brexit EU summit British Prime Minister David Cameron arrives during the second day of the EU Summit as he continues to negotiate new membership terms for the U.K., at the Council of the European Union in Brussels, Feb. 19, 2016. Photo: Getty Images

Although Johnson is a member of Cameron’s Conservative Party, he draws support beyond party lines, often using populist rhetoric that supports libertarian fiscal policies and a smaller government, while mostly remaining socially liberal. Elected as London mayor in 2008, he has kept many people guessing as to his position on a Brexit, making his announcement Sunday in an interview outside his home.

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,” Johnson said Sunday, the Washington Post reported.

Cameron has worked closely with Johnson in the past and even named him as one of his three picks for a successor in 2020. Johnson is the only one of the three who has not supported the U.K. remaining in the EU — and his endorsement of the pro-Brexit campaign bears weight, according to a recent poll. Nearly one-third of U.K. citizens who responded to an Ipsos/Mori poll said their vote would be influenced by the mayor’s position, and around 44 percent said Cameron would influence their choice.

If the U.K. ends up voting in favor of a Brexit , Cameron may step down, allowing Johnson to vie for prime minister. The mayor has already alluded to his ambitions for the prime minister position in interviews, and preliminary projections give him good odds. Johnson overtook Chancellor George Osborne as the favorite to succeed Cameron for the first time, the Telegraph reported Monday, citing odds from the booking agency Paddy Power.

“Johnson is a national brand name in the UK and has considerable international recognition as a popular mayor of London, a leading global city.  Among all the alternatives to David Cameron in the Conservative Party, Johnson has skillfully crafted a leading position for himself,” Marco Vicenzino, director of geopolitical risk and international business advisory firm the Global Strategy Project, said in an email interview Monday. He added, “However, it is important not to underestimate David Cameron.”

A transition out of the EU is slated to be a rocky one, and a fiscal disaster spurred by a Brexit would harm Johnson’s chances in 2020. The Moody’s credit ratings agency said the costs of an exit would outweigh the benefits, the Press Association reported Monday, noting that uncertainty could be devastating to foreign investment.

“They don’t actually have a clear template for what Britain would look like outside the EU,” Iain Begg of the London School of Economics said of pro-Brexit platforms. “So in that sense it’s a step into the unknown.”

Johnson Mayor of London Boris Johnson announced he will back the “Leave EU” campaign while speaking to the press outside his home, Feb. 21, 2016. Photo: Getty Images

The fate of London in particular has been a point of contention in the Brexit run-up, as much of London is pro-EU and Johnson has been an outspoken euroskeptic. “He’s conflicted basically between being skeptical of the EU, and the city wants to stay in the EU,” said Waltraud Schelkle, a political economist at the European Institute of the London School of Economics. “So his problem is to go between these two things.”

While some anti-EU campaigners in the capital city have argued that the large financial hub would remain viable with or without the EU, several economists and political experts have criticized those claims, saying Johnson’s city would in fact see some of the harshest fiscal consequences of a Brexit .

“London is going to be worse hit than other places,” said Swati Dhingra, an economist specializing in trade at the London School of Economics, describing the city’s deep connections with European trading partners and foreign investors.

“When your bottom line starts to get worse, you might start to see businesses move to Paris, Frankfurt,” she said.