Tuesday night’s televised debate on ITV between Jeremy Hunt and Boris Johnson is the only chance the public will get to see the two candidates to succeed Theresa May argue their points head-to-head. But it won’t change anything.

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It had been delayed, by Johnson’s team, until the ballot papers had already gone out and in many cases already been filled in. There is plenty of evidence that the 160,000 Conservative members – a tiny proportion of the whole UK electorate – have long ago decided to put Johnson in No 10; some of them joined the party specifically for that purpose.

And Brexit, the central issue of the day, is an article of passionate faith and dogma among most Tories, not an issue where strategies can be weighed and considered for their prudence and consequence.

Yet the debate was revelatory nonetheless. We can’t now say we haven’t been warned about what a Johnson premiership will involve. On the contrary. His statements and his silences told the story unambiguously.

He won’t quit if he fails to deliver – as he will fail – on his central promise to get Britain out of the EU by 31 October. He will try to prorogue parliament in order to drive Brexit over the line if he thinks he can get away with it.

He will sack Sir Kim Darroch as UK ambassador to the United States as soon as he gets his feet under the table and he will cosy up to Donald Trump by making a political appointment to succeed the man Trump called “wacky”.

He will not override the Democratic Unionist party on LGBT rights in Northern Ireland. The HS2 high-speed rail plan is now as good as dead. And Johnson will kill the third runway at Heathrow and, in all likelihood, revive his plan to move London’s airport to an eye-watering and uncosted site in the Thames estuary which Britain’s sycophantic press will undoubtedly christen Boris Island.

Johnson won the debate because he is anyway going to win the ballot among the predominantly rightwing, old, white and male inhabitants of southern England who make up the electorate.

But Hunt won the evening and the contest because he fought a smart and undeterred fight that consistently highlighted Johnson’s copious flaws. While Johnson bragged and played the Oxford Union smart arse, Hunt got through his defences with detail and well-placed barbs.

Hunt was the only one to talk about the danger of the UK breaking up. He defended May against Trump. He was the only one to talk about low pay and the only one to talk about the “very serious” dangers of a no-deal Brexit. You can’t get through “on a wing and a prayer”, said Hunt. But we all suspect that this is exactly how Johnson is likely to win. Hunt gave it his best shot, but it won’t turn the contest his way.

In political debates, optimism mostly beats pessimism. Johnson was good at optimism. He radiates self-confidence and ego. The audience seemed in his favour, though these things can be deceptive. He finished by appealing to the can-do spirit even though most voters are likely to suspect it is a fantasy.

He repeatedly condemned Hunt as a defeatist. Hunt countered that being prime minister was about telling people what they needed to know, not what they wanted to hear. But this is not a good message in a contest in which the Conservative leavers want passionately to hear that Brexit will happen by the autumn, and Johnson gave them what they wanted.

The most useful part of the exchanges came with the quickfire questions towards the end. Hunt had clearly decided that he needed to show decisiveness. He gave one-word answers, very effectively. Johnson waffled, avoiding giving answers of any kind, but his evasions were eloquent, too. The smirks, the smugness, the self-love, the bluster and the hyperbole told us that Britain is about to be landed with the most ill-qualified and most insouciant prime minister in modern times.

He may not last long in office, which would be a relief for a damaged country. But none of this is going to make any difference to the result.

• Martin Kettle is a Guardian columnist