Until quite recently, it was still accurate to say that the central problem in British party politics was Brexit. This week, that has imperceptibly but now decisively changed. Now, and probably for much of the coming summer, the central problem in British party politics is no longer Brexit itself but the character of the post-Theresa May Tory party.

May’s decline from the now hard-to-recall autumn of 2016, when she enjoyed the confidence of 87% of Tory voters and 54% of the whole electorate has been long and slow. The mishandling of the needless election of 2017 resulted in enormous self-inflicted damage. But even as the diminished May continued to battle on into the early part of this year to get her withdrawal agreement adopted, British politics was still essentially about Brexit rather than her.

That is no longer the case. Brexit has not gone away, but it has become a subplot to the battle for the Tory party. May’s own mistakes and the fanatical obsession of the rightwing of the Tory party that Brexit can be solved by a different leader have combined to destroy her last room for manoeuvre. Brexit is consequently stalled, its long-term outcome more than ever uncertain – which may be a good sign for pro-Europeans.

If the situation facing Britain was not now so serious, there would be an irony in the approach that May has adopted

Meanwhile, the prime minister who launched a “new, bold” Brexit withdrawal plan at the start of the week is now struggling to publish it on Friday, and even to survive. If, in tomorrow’s European elections, May’s party scores the abject and record-breaking 7% score that a YouGov poll suggested this week, she will have to go, and not unreasonably. Fear of such a humbling may be the proximate cause of the flurry of efforts to oust her today. But it is surely too late to change the outcome in the elections.

In the Commons today, May went through the motions of launching her new withdrawal package. It was a lacklustre session. Almost all the passion and drama of the early spring on Brexit is now spent. The Tory action was going on elsewhere. Andrea Leadsom’s resignation tonight was a piece of specious opportunism; it will not be the last, as leadership contenders jostle to establish the high-minded purity of their motives. May ploughed on, because that is what she does. There was no one to say, as Leo Amery did so ringingly to Neville Chamberlain in 1940 – “In the name of God, go.” But that was the moment that May has reached all the same.

If the situation facing Britain was not now so serious, there would be an irony in the approach that May has adopted as her power drained away. If she had reached out to the opposition in 2016, and done it well, she would probably have come up with a package not unlike the one she is promoting at the last. Two and a half years ago, that package would quite likely have won the day, and May today might have been the leader who had brought the country out of the EU with a tolerable future relationship with the 27 in the making, and undisturbed by even a distant thought about Nigel Farage.

Instead, with Nick Timothy pushing her in an ever more uncompromising Brexit direction in the decisive early months of her premiership, May did the exact opposite. That is all history now. But it was fateful history, and a self-absorbed, increasingly messianic and dogma-driven Tory party has still not absorbed the large lesson from it; that this once broadly-based party is well down the way to becoming a narrow-minded and embittered English sect which, among other things, may be about to trigger the splintering of the United Kingdom itself.

It left May now doing the right sort of thing but badly and at the wrong time, when she no longer commands her party or frightens the opposition. It is still the case, even now, that if the Tory party really wanted to kill off the Brexit party, it would rally behind May’s bill, give it a second reading and allow the prime minister to step aside with some vestigial dignity before the autumn. Britain would be out of the EU. Brexit would have happened. And only a cluster of obsessive Tory Boys in suits would then be preoccupied any longer with things like the customs union, the transitional period and the backstop. The air would go out of the Brexit party and Farage would return to spend more time with his money.

May cannot be let off her own share of responsibility for the situation that she and we have reached. She has done the politics of the last three years extraordinarily ineptly in many ways. One of the small but significant things she mishandled recently was to allow MPs to have a break over Easter. It was an understandable human gesture, but in retrospect she should have kept the Commons’ noses to the grindstone until they came up with a deal. Instead, she allowed all the tension to dissipate and she has not been able to re-establish it.

But the Tory party must shoulder its share too. Once again, the post-imperial nostalgic men and the unreconciled Thatcherites have proved that they have the power to dethrone a Tory leader. Like Thatcher, Major and Cameron before her, May’s prime-ministership has been wrecked over Europe. But whereas her three predecessors all had other things in their legacies, May has just the one – Brexit. Hers was the Brexit premiership, and her defeat is inescapably a victory for the Brexiters, even if they still have no clear idea what – beyond a platonic version of Brexit – they want to put in place instead.

The capture of the Tory party by this ideological, nationalist, overwhelmingly English part of the party has gone further than ever before. It must be genuinely doubtful if there is a way back any time soon. For many years, one of the great “What if?” games of British politics was to ask what would have happened to Britain’s place in Europe if Michael Heseltine had won the prime-ministership in 1990, as he so nearly did. Above all, would he have left behind a Tory party in his own mould on Europe? And, if that had happened, would the Eurosceptic tide have been as successful in shaping Tory policies as it was?

All one can say is that, to this day, Heseltine remains a leonine voice for the way not taken. That he has had the Tory whip removed for saying he will vote for the Liberal Democrats tomorrow is a mark of how it’s the Tory party, rather than him, that has changed. George Osborne’s editorial for the London Evening Standard today was another sign. It’s not just the Heath-era one-nation Tories like Heseltine who now have no home in the Tory party. It’s the Cameron-era one-nation Tories too. The new One Nation declaration of values this week written by Tory MP George Freeman – and endorsed by Amber Rudd and Rory Stewart – is a fine document, but it looks more like the manifesto of a new party than anything the modern Conservative party has any wish to embrace.

This is a change moment. The Tory party has brought it entirely on itself. If the results this weekend are as bad as they expect, a combination of panic plus grassroots pressure could propel the Tories to embrace the Brexit party, not shun it. The consequences of that could be immense. The Tory party, like Labour after 2015, is wide open for a takeover by a surge of new members. To believe that May’s overthrow will mark the high tide of this new rightwing nationalist Toryism is absurd.

• Martin Kettle is a Guardian columnist