The Leave campaign now has a powerfully persuasive standard-bearer

Boris Johnson’s decision to argue that Britain should leave the European Union has suddenly made David Cameron’s task of winning the referendum on June 23 a good deal harder. As one of the three best-known politicians in the land alongside the Prime Minister and George Osborne, Mr Johnson’s recruitment to the Out cause has electrified the campaign as no one else could.

His populist pedigree has been evident for the past eight years in London, where he won two elections in a city where Labour would normally have expected to succeed. He brings to the Leave side a sheer force of personality that it would not possess without him. This can only be good both for the debate and for democracy: Mr Cameron is by no means going to have everything his own way.

Mr Johnson has been accused of dithering; but he has, in truth, been grappling with the same conflicting views that many people will find hard to reconcile in the coming months. After all, it is perfectly reasonable to have been a long-term Eurosceptic, unhappy with the direction being taken by the institution, without necessarily believing Britain should leave.

But in his case, if he was going to make a credible argument to stay, Mr Johnson had wanted the changes that Mr Cameron promised to be more farreaching than they turned out to be. Moreover, in stating that the package was insufficient, Mr Johnson has undermined the very foundations of the Prime Minister’s case – of securing a new future for Britain in a reformed Europe.

It is noteworthy that having battled so hard for the terms of the settlement agreed on Friday, Mr Cameron has now effectively conceded that it is no longer relevant to his cause by emphasising other arguments for staying in. Interviewed on The Andrew Marr Show yesterday, the Prime Minister gave a combative performance that demonstrated what a formidable advocate he will be for the Remain camp. But he moved quickly from the minutiae of his hard-won Brussels deal to focus on the bigger picture. Mr Cameron maintained that Britain is safer inside the EU and its prosperity was linked to membership. We could, he averred, have the best of both worlds by remaining in the European club while abiding by a different set of rules (or at least by a smaller set), by staying out of the euro and Schengen, and by being exempt from the ambitions of “evercloser union”. In addition, the Prime Minister believes he has protected the City of London from the predations of our EU partners who resent its power and reach.

If we stay, said Mr Cameron, at least we know what is on offer; if we leave, everything is uncertain – we will be taking a “leap in the dark”. This will be the most powerful argument to be deployed by the Remain side to convince voters in what will be the most important decision this country has taken since the referendum in 1975.

As it proved in Scotland in 2014, the strongest case for the status quo is fear of the unknown. The principal task for the Outers, therefore, is to address this point directly by showing what Britain would look like outside the EU. With articulate champions such as Mr Johnson and Michael Gove on their side, the Leave campaign is now even more formidable than Downing Street feared.

Far from being a coalition of the usual suspects like Nigel Farage, longstanding Tory and Labour Eurosceptics and mavericks such as George Galloway, it has an appeal broader than anything the anti-marketeers could muster in 1975. Then, all the main party leaders, most MPs, business chiefs, the judiciary and the media were four-square behind staying in. This time, even if the Leave side likes to give the impression that it is a minnow taking on “the Establishment”, the forces are more evenly balanced.

Those on the Out side who felt they would be steamrollered by a wellorganised and expensively funded campaign now have some of the most powerfully persuasive voices on their side. It is disappointing, therefore, that Mr Johnson has expressed reservations about debating face to face with his fellow Conservatives. The country will expect the principal protagonists to test their arguments against each other at some point, which is why we stand ready with our partners Google and Buzzfeed to facilitate such a debate.

Most important, though, is that now we know the date of the referendum, the terms of the renegotiation and who stands where, the Leave side resolves its differences and unites around a single message with a single leader. We asked a few weeks ago who would step forward to take up the battle standard on behalf of the Outers and Boris Johnson has done so. June 23 is his date with destiny.