Much has been made or said about the ‘clean campaign’ pledge signed by various if the Tory leadership contenders this week. The first rule transposes what Ronald Reagan called his ‘Eleventh commandment’: “Thou shalt not speak ill of a fellow Conservative”. This is essential to ensuring the new leader commands the confidence of the party and the country. But there’s another pledge that ought to be even more important in making sure the party comes together with a new sense of unity and resolves the challenge facing it. And that is: everyone standing should back whatever Brexit outcome is decided on by the members’ choice for the next Prime Minister – and urge all of their supporters to do likewise. Whoever is elected leader of the world’s oldest political party inherits an unenviable task and unyielding parliamentary arithmetic. A change of Prime Minister does not – in and of itself – change that arithmetic, with Conservatives still dependent on others to get their business through. Equally, they are beholden to others not to out-vote the Government. Parliament has undergone what Sir Bill Cash has called a ‘constitutional revolution’ in now giving unprecedented power to the Opposition to control parliamentary time. As such, even though No Deal is the legal default, it’s not within government’s control to exit without a deal without parliamentary acquiescence. This is a significant problem for a Conservative Party that clearly needs to deliver on the referendum result – and ensure the UK leaves the EU by the end of October – in order to survive. To get there, the next Prime Minister has to deal with ‘Spartans’ on both flanks. At the last Meaningful Vote, 28 eurosceptic Conservatives voted against it, thinking that doing so would lead to a cleaner Brexit, while six anti-Brexit Conservatives voted against it, thinking it would lead to a softer Brexit. Clearly, both camps can’t be right, but the absence of any Brexit hurts both factions and hurts Brexit itself. That vote failed by a majority of 58, so even if the deal had been amended such that every eurosceptic Conservative had backed it, it would still have been defeated. As such, both factions must budge for any arrangement of any sort to pass. The arithmetic could, of course, get worse for No Deal – with many who backed the Withdrawal Agreement, like Philip Hammond and Rory Stewart, saying they’d oppose it. Regardless of the arrangement a new Prime Minister comes to – whether a new deal, a decision to exit with No Deal or something else – it is unlikely that both factions would naturally like it, no matter what arguments are made, as their positions are too entrenched. However, this can be avoided if participants in the leadership election commit now – before knowing what the outcome will be – that they will endorse the outcome of the leadership election, back the Brexit policy pursued by the winner and urge all their supporters to do likewise. This is committing to a form of binding arbitration, with the arbitration in the hands of party members electing the leader. Right up until the point the Prime Minister has decided the Brexit outcome, MPs would be free to make the case for his or her preferred outcome (hopefully not publicly in the case of ministers!). However, by the time a decision is arrived at, all – from whatever flank or faction – should already be committed to respect that result. This strikes a fair balance between the right of members to set party direction and the right of MPs to use their judgment, while ensuring the public’s vote of 2016 is delivered on. Conservative MPs have unparallelled power in the leadership election process and far more say in it than those across the aisle in other parties. With that added power comes a solemn duty to support the decision that Conservative members then come to in deciding the final victor: much as those that voted Remain must respect the referendum outcome. If MPs don’t respect the outcome, and they insist on making the perfect the enemy of the good, no outcome will pass. This would mean no-one getting their way on Brexit: the deadlock would continue and the can kicked further down the road – and the Conservative Party would likely cease to exist, all because MPs were playing a game of chicken. A pact now that everyone standing or voting in the leadership election will put their own preferences aside and back whatever the Conservative Government ultimately decides to do would end that game of chicken. That is more important than any other pledge or policy that any candidate can unveil.