London Mayor Boris Johnson backs ‘Brexit,' boosting anti-E.U. campaign

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

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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 almost certainly will be intense pressure on Cameron to step down. And Johnson, having seized the populist moment, would be uniquely positioned to take his place.

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Until Johnson's announcement, only relatively marginal political figures had taken up the cause of 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.