Boris Johnson has been sliced and diced. The real winner is Rishi Sunak

It had felt tempting beforehand just to look away, and by doing so mount your personal, not-in-my-name protest against the Boris Johnson show’s latest attempt to hijack the battered and drifting hulk that is British politics.

But this country is too far gone for any lingering squeamishness about Johnson to be permissible. The issues that surround him are too important, the dangers of ignoring him too great. What mattered on Wednesday was that a stake should be driven through the heart of the beast. And, to a quite unexpected degree, it was.

Many Conservatives – and plenty of rightwing journalists – will have hoped that this would be the day Johnson launched his recapture of the Tory party from Rishi Sunak. Many other Tories will have feared the same thing too, if only because of the disruption the attempt would cause Sunak, even if Johnson’s bid ultimately failed. What neither group could have fully foreseen, as night fell on Westminster’s stormiest political day this year, is that Wednesday was close to being a disaster for the former prime minister.

It was a botched hijack for two main reasons. The first was the way the Commons privileges committee confounded any assumptions that they would be partisan pushovers. Instead, the MPs calmly and devastatingly sliced and diced Johnson and the evidence about Downing Street Covid gatherings he has given this week, in print and now in person. If Johnson thought he was among friends, he was soon to discover the opposite.

At first, as he read from a long statement prepared by his lawyers, things started well for Johnson. He was in combative mood, keen to mix it with the committee chair, Harriet Harman. Once Johnson no longer had a script to speak from, and was instead forced to think and speak on his feet, things went rapidly downhill. The unexpected star turn here was Sir Bernard Jenkin, a Conservative MP whose Brexit credentials are unchallengeable, who quietly carved Johnson’s evidence into pieces, leaving him spluttering and humiliated.

The second was Johnson’s politically self-destructive decision to vote against the government’s Windsor framework Northern Ireland proposal. The early-morning announcement of that move triggered excited speculation that Sunak would face the humiliation of a large Tory revolt that would require him to rely on Labour votes. But Johnson had misread the room. In the end, only 22 Tory backbenchers voted against the government. A potential humiliation for Sunak was transformed into something of a triumph.

It also left Johnson becalmed among ageing doctrinaire fanatics such as Bill Cash, Chris Chope, John Redwood, Jacob Rees-Mogg and Mark Francois, and more isolated than at any time in the past decade from the Tory mainstream. Wednesday’s voting sends a powerful message to more pragmatic Tories that Johnson no longer offers them the electoral escape route that he conjured for them in 2019. The magic – or the poison – has gone.

By the same token, it also strengthens Sunak. The prime minister had a good day. Until now, his party leadership has been highly contingent. He only got the job because Liz Truss destroyed her own government so quickly. Sunak has always had Johnson’s shadow hanging over him. Now, though, the momentum is with Sunak. It may not save the Tories when the election comes. But, after Wednesday, Sunak has no reason to fear Johnson. He can even target him, perhaps by blocking Johnson’s honours list for being the corruption that it is.

In a strict sense, today’s session in Westminster’s Grimond Room was simply a public hearing during an inquiry into whether Johnson consciously misled parliament. Laugh, by all means, at the absurdity of supposing there can be any real doubt about that. Mock, if you wish, the semantic squabbles about whether the greased piglet’s actions and words were inadvertent, reckless, intentional or deliberate.

But don’t be misled into dismissing this inquiry as arcane, or as a piece of petty parliamentarism, not real flesh-and-blood politics. That would be terribly, terribly wrong. In procedural, and indeed in moral and historical terms, this inquiry matters a very great deal. A lot hangs on it for Britain. What hangs on it is not merely Johnson’s tattered claims to be an honourable public figure. It is the survival of our representative democracy in an age of demagogic leaders who despise parliamentary norms.

Parliament is the sovereign apex of the nation’s democratic government. Its credibility depends upon the executive telling the truth to MPs, and through them the nation. If Johnson did not do that, especially in the circumstances of a killer pandemic in which obeying the rules was so paramount, he must pay the price.

It is no excuse to claim that they all do it. True, modern politics routinely operates by elbowing truth aside and battering trust into submission. No politician is untouched by these habits, never mind Johnson, who, as he displayed in his exchanges with Jenkin, is psychologically incapable of providing a straight answer to a straight question.

Johnson, though, was also prime minister during an unprecedented peacetime lockdown. Ultimately, that’s one big reason why Wednesday mattered so much. He was the head of a government that told the rest of us where and under what conditions we could associate with others. Lives were at stake, thousands of them.

Related: Simon Case denies telling Boris Johnson Covid rules were always followed

For Johnson to have been anything less than assiduous in following his own rules, and anything other than meticulous in accounting for his and his office’s conduct, put the national effort at risk. Even someone as licentious and morally incontinent as Johnson must have grasped this at some level.

That he ought to have erred on the side of strictness – in the way that the Queen and so many millions of others so visibly did – seems never to have occurred to him. Jenkin’s cross-examination pinioned him on this issue – and revealed that even now Johnson doesn’t get it. This is why, in the end, the privileges committee process must drag him further down, not build him up again as he hopes.

After Wednesday’s committee hearing, it is more likely than before that this will all play out the way that Johnson’s opponents want. In that event, the committee will produce a united conclusion that he broke the rules by the way he misled parliament and by acting too slowly to correct his falsehoods. MPs will then vote to suspend him for 10 or more sitting days. A recall petition will be triggered in his Uxbridge and South Ruislip constituency to demand a byelection, in which Johnson will be ousted from the Commons.

Well, perhaps. There are, though, plenty of ifs and buts in that scenario. The most politically important of them concern the Conservative party. Most Tory MPs probably woke up today hoping they could find a way of sparing Johnson a long suspension. After his performance at the committee, they will have gone to bed less confident about doing that. Johnson has proved that he is no longer any part of the solution to the Tory party’s woes. Instead, and more clearly than ever, he is part of its problem.

• Martin Kettle is a Guardian columnist