There was a joke made at the expense of Nick Clegg when he performed his disastrous and, for him, ultimately terminal betrayal of his tuition fees promise at the outset of the 2010-15 Coalition Government. And I must confess that I might have had something to do with it. The joke was that the only mandate Clegg had was a cosy dinner with David Cameron (“man-date”, geddit?). But bad jokes aside, mandates are key in politics. They bestow democratic authority on those who receive them and drain it away from those who do not. And this is why Boris Johnson’s renewed onslaught against the Chequers plan matters so much. Those MPs who backed Remain in the referendum can throw as much abuse or sarcasm his way as they like – from Sarah Wollaston calling him a charlatan to Sajid Javid’s patronising chortling at the expense of the blond bombshell – but an awkward fact hovers over their every word. It is that Boris Johnson led the official Leave campaign to a stunning popular mandate bestowed by 17.4 million voters. His detractors faced him in a great democratic battle in which they had all the advantages of establishment backing and the status quo ante on their side and yet they were comprehensively bested by him and his cohorts. It is because of Boris and a very few other big figures that the Government’s core mission is so defined. Had it been up to Theresa May or Philip Hammond or Jeremy Hunt or Javid there would have been no Brexit at all. They did not believe in it – or in Javid’s case at least not enough to campaign for it. So they cannot claim to have vested in themselves the mandate of the people to see it through. The best that can be said for Mrs May and her team is that they have the buggered-up, partial mandate of the botched and unnecessary 2017 general election behind them. But as Remain ultras never tire of pointing out, Mrs May did not win that election; rather, she lost her Commons majority in it. It is largely the fact that Jeremy Corbyn offered such a pro-Brexit manifesto of his own in that contest that kept fully intact the authority of the 2016 referendum. Between May and Corbyn and their offerings there is a powerful composite mandate for full Brexit. But measure May’s democratic authority to shape the course of Britain’s departure from the European Union against Boris Johnson’s and the former is sadly lacking. So when Johnson writes that Chequers is a white flag policy and does not remain true to the Brexit he campaigned for and Britain voted for, it is a devastating charge. He is speaking for the vast majority of Leave voters. Polling on Chequers confirms that. But he is also speaking with the direct authority of the Brexit mandate behind him. This mandate was not an inconclusive general election based on many different issues, this was the largest exercise in democracy ever carried out in our country and it was on the specific issue of Brexit and Brexit alone, it produced a clear result and the case for it was communicated primarily by Boris Johnson. Certainly Boris’s authority is shared with other prominent Leave figures, some of whom are still lurking in the Cabinet and keeping their heads down rather than going out to bat for Chequers on a regular basis. But Boris was the figurehead of Vote Leave and Boris – along with Nigel Farage, David Davis and other implacable opponents of Chequers – is the one speaking with the grain of Leave voters. The most important thing that flows from this is that in all likelihood Chequers is dead. If the Tory Party allows May to reach a deal with the EU based on Chequers and the common rulebook and all the rest of it, then it will be seen to be imposing a fake form of Brexit against the wishes of Leave voters. For a party that depends on Leave voters for 75% of its support, that would be a pretty stupid thing to do. Voters will have noticed by now that there has been a sea change in the nature of the Prime Minister’s support within the Tory Party. The senior figures cheering her on now are not those who campaigned for Brexit – as they were a few months ago – but those who campaigned against it and even those like Ms Wollaston who show every sign of still wishing to stop it happening at all. Mrs May is no longer the Remain-backing Home Secretary who became a Leave-backing Prime Minister. She is rather doomed to be seen as the Remain-backing cuckoo in the nest at 10 Downing Street – an imposter who stole the Brexit mandate by making false promises and is now having it taken back from her by Johnson, Farage, Davis and its other true keepers. To end in a similar vein to that in which I began, her only reliable mandate is supper with Philip in the flat above Downing Street. And this is a very much more fundamental matter than student tuition fees, terminal as they proved for Mr Clegg. If she won’t ditch Chequers then the Conservative Party must ditch her.