Will Boris Johnson compromise on Brexit? As the frontrunner edges closer to Downing Street, that is the hope, if not the expectation, of a fair number of his supporters in Parliament. They believe that, once safely in office, Johnson will end up delivering what Theresa May couldn’t – that is, the existing (and thrice rejected) withdrawal agreement with a few tweaks.

Those assumptions were always more likely to be wishful thinking than not, and, addressing the last head-to-head debate of the campaign this evening, Johnson took another sledgehammer to them. Asked whether he would agree to a five-year time limit or unilateral exit clause for the Irish backstop, the border insurance policy that was ultimately Theresa May’s undoing, his answer was a categorical no.

His disavowal of both of those possible compromises – which, it should be said, have been repeatedly rejected by both Dublin and Brussels and are already the least likely concessions they could plausibly offer – is hugely significant. Why? Throughout the entire Brexit process, the only form of withdrawal that has commanded a parliamentary majority made up of Tory and DUP MPs alone is a compromise along those lines: a withdrawal agreement either gutted of the Irish backstop, or with its backstop neutered.

It is less clear whether those Eurosceptic MPs who would have – and did – accept that sort of compromise before 29 March would do so now, given their new embrace of no-deal as a first, rather than last, resort. But even allowing for a little slippage of support at the fringes, it is still just about the only outcome Johnson could have proposed while keeping his parliamentary coalition intact.

In rejecting it so unequivocally, Johnson has exponentially increased the chances he will default to a no-deal position – and closed off one of the only potential exits from his collision course with opponents of no-deal on the Tory backbenches. A withdrawal agreement with no backstop, which by his logic is the only acceptable negotiated Brexit, is a non-starter as far as Dublin is concerned. Tonight he has, in essence, turned down the only negotiated withdrawal on offer from the EU.

He has sharply limited his room for maneouvre should he end up wishing to avoid a no-deal scenario too. As of this evening, his position is that any withdrawal agreement with a backstop – regardless of legal tinkering – is a compromise too far. Should he present anything like that to the Commons, he will be inviting mutiny from the hardline Brexiteers who, as they are keen to stress, have only lent him their support.

As with so much of Johnson's campaign strategy, any political capital tonight's gambit generates will be worthless as soon as he enters Downing Street. Not only has he increased the chances of a destructive confrontation with his own party over no-deal, but he has made the politics of doing a deal – something he insists he still wants to do – so tricky and toxic as to effectively rule it out.