After the interminable teasing, the winks, the nudges, it would have been humiliating for Boris Johnson to announce that he was, after all, supporting EU membership. “Bottler Boris” would have been the epithet of choice.

By instead declaring for Brexit, the Mayor of London has made the choice that will best serve his primary cause - becoming Conservative leader and prime minister. Tory MPs testify that Johnson has long been privately supportive of EU membership ("The trouble is, I am not an 'outer,'" he is said to have told them late last year). Unlike Michael Gove, who declared in 2013 that he would vote to leave on the current terms, the mayor has never publicly advocated withdrawal.

Though David Cameron’s renegotiation was not trivial, the stakes were too low to determine Johnson’s choice. His prevarication transparently stemmed from uncertainty over which stance would enhance his leadership ambitions (though it would be wrong to assume that considerations of principle played no part). While the Conservative grassroots overwhelmingly favour Brexit, the polls point to a victory for In. Can a putative prime minister afford to be on the losing side?

The answer turns out to be yes. Johnson has rightly calculated that backing Out is a win-win for him. Should the UK vote to leave, David Cameron will almost certainly resign as prime minister and the mayor will be best placed to succeed him (the referendum has become a proxy leadership contest). Should the country vote to remain, Johnson will nevertheless have earned the affection of the Tory tribe and differentiated himself from his rivals (George Osborne and Theresa May).

The grassroots will not relinquish their antipathy towards the EU merely because the public have voted to remain. As the Scottish experience demonstrated, the referendum will more likely sharpen the European question than settle it. It was in 1981, just six years after the 1975 vote, that Labour endorsed EEC withdrawal. Whenever the Conservative leadership contest comes, opposition to the EU will almost certainly be an asset (witness the surge of Liam Fox). Gove's decision gave him valuable political and intellectual cover.

By backing Out, Johnson has heightened the chance that the public will take the same view. For years he has enjoyed the status of the country’s most popular politician - there is no figure that the In campaign fears more. An Ipsos MORI poll last week found that he was second only to Cameron as the politician with the greatest influence over voters. For a campaign comically lacking in popular leaders, with the Marmite ensemble of Nigel Farage, George Galloway, Gove and Iain Duncan Smith, Johnson’s endorsement is a coup. It may prove to be one for him too.