Before Christmas I reported that a cabinet minister could come out for EU withdrawal. Today, Chris Grayling, the leader of the House of Commons and the name tipped to me at the time, has all but done so. In a piece for the Telegraph, he writes that "I am someone who believes that simply staying in the EU with our current terms of membership unchanged would be disastrous for Britain". He goes on to signal that David Cameron's limited renegotiation will not satisfy him.

It's safe to say that the In campaign has been untroubled by Grayling's intervention (head of communications James McGrory sardonically tweeted: "If Chris Grayling is a 'senior' Cabinet Minister, I am an aubergine). But there is one figure who gives pro-EUers sleepless nights: Boris Johnson. In recent conversations with senior figures, the Mayor of London has been consistently cited to me as the potential Brexiter they fear most - and rightly so. Polls have consistently shown him to be the most popular politician in the country and no Conservative is better at appealing to voters beyond his tribe. With David Cameron the de facto leader of the In campaign, the Outers need a figure capable of matching him - and only Johnson fits the bill (EU supporters are less troubled by Theresa May). He would imbue their campaign with wit and optimism, making it far harder to cast those wishing to leave as sour little Englanders. Perhaps Johnson's greatest talent is his ability to make voters feel good about themselves. As the In campaign stresses the dangers of leaving (for the economy and national security), the mayor could convince the public that Brexit is nothing to be frightened of.

The hope of the Out campaign is that Johnson will side with them to differentiate himself from George Osborne (his chief leadership rival) and win over the anti-EU Tory grassroots. But the smart money is increasingly on the mayor siding with In. He will reportedly be offered the post of foreign secretary (or another senior cabinet job) in return for backing EU membership. Before Christmas, he is said to have told Tory MPs: "The trouble is, I am not an 'outer'".

Though the mayor has been viewed as an ardent eurosceptic ever since his days as the Telegraph's Brussels correspondent (when he spawned a new genre of EU-bashing journalism), others emphasise his pro-European roots. His father, Stanley, who worked at the European Commission from 1973-79, once remarked: "Boris is a very good pro-European. He basically had a perfectly good European upbringing, he went to the European School. Don't tell me he is not. You can't be called Alexander Boris de Pfeffel Johnson without having a European vocation. Honestly." On another occasion, after Michael Gove said he would vote to leave based on the UK's current terms, Johnson said: "Most of our problems are not caused by 'Bwussels', but by chronic British short-termism, inadequate management, sloth, low skills, a culture of easy gratification and under-investment in both human and physical capital and infrastructure."

The mayor does not have long left to make up his mind. Should Cameron's renegotiation conclude next month, as hoped, he will finally be forced to show his hand. No politician's decision will do more to determine the outcome of the race.

Now listen to George Eaton discuss this subject with Helen Lewis, on the New Statesman podcast: