David Cameron has announced that the referendum on whether Britain will leave the EU is to be held on June 23. This marks the beginning of a four month campaign that will have enormous repercussions for his country, his party and his own legacy as prime minister.

Cameron left gruelling negotiations in Brussels with a triumph that he claims has resulted in Britain’s concerns being addressed and the sovereignty of Britain being assured. The country now has “special status” in the EU, he said after the meeting with fellow national leaders.

So far, so predictable. There has never been any doubt that Cameron would emerge with an agreement. The alternative was to recommend Britain leave the EU – something the PM could never have done.

One of the rules of EU negotiation is that everyone has to win. Whatever the agreement, politicians turn to their national media to claim that they have secured a great deal. In normal circumstance this is accepted and people move on and forget about it.

But the same rules don’t apply back home in Britain. Now, following a meeting with his cabinet ministers, several high-profile figures have confirmed that they will campaign for a Brexit. So far cabinet ministers Chris Grayling, Theresa Villiers and the Prime Minister’s friend and ally, Michael Gove all confirmed they would be joining the Leave campaign.

Home secretary Theresa May and business secretary Sajid Javid have decided to campaign to remain alongside their party leader.

Cabinet split

With British voters unlikely to be particularly swayed in either direction as a result of this deal, the negotiation was to a larger degree about the Conservative Party.

The decision to hold a referendum at all was an attempt to limit the endemic conflict within the party over Europe. Cameron knew that he could not recommend a yes vote without presenting a renegotiated membership.

This deal won’t please the hardline eurosceptics, but then no deal would. The goal was to win over the big beasts such as Gove and Boris Johnson. Cameron’s failure to convince key ally Gove provides the leave group with a senior figure to galvanise the campaign. The problem for Cameron is that the leave campaign is no longer focused on divisive outsiders like Nigel Farage and George Galloway but has senior members of the Cabinet involved too.

Johnson’s part is equally important. His slow political dance has kept everyone guessing about whether he will campaign for in or out, almost overshadowing Cameron in the process.

Your move, Boris

As he so often does, Johnson has played almost pure politics. He is known for his euroscepticism and made it clear that he didn’t think much of the deal on the table in Brussels before it was agreed. But he is also the mayor of multicultural London, whose virtues he extols and whose economic success depends on a financial centre that is committed, on the whole, to remaining in Europe.

Johnson has managed to wring every ounce of attention out of his position of ambiguity. Even now, his position is being decided not on the substance of the deal but on the political reaction. Will Cameron win the referendum? And how much support will Johnson lose in the party by backing a Yes vote? Johnson will calculate the best outcome for him personally.

If the British people vote to leave in June, Cameron will have little choice but to resign and Johnson is in pole position to take over as leader.

Of course, Gove’s decision to support exit makes Johnson’s position more difficult. If Gove is the senior representative of the out group and Johnson sides with Cameron, the mayor’s chances at the leasership are diminished. Johnson now faces a crucial question – should he follow the prime minister or Gove?

Cameron is starting a long campaign on the back foot. For the next few weeks, the discussion will focus on the negotiations and what they mean, rather than the benefits of British membership. Meanwhile, anti-EU Conservatives have an opportunity to claim the high ground by focusing on the failures of the negotiation, as Grayling has done already.

Cameron has had a tough few days, it looks like the coming weeks are going to be tougher still. Not only is British membership of the EU on the line, but so is his future.

Martin Smith receives funding from the Home Office/HEFCE and the ESRC .

Martin Smith, Anniversary Professor of Politics, University of York

This article was originally published on The Conversation. Read the original article.