Life is full of unexpected symmetries. In 1996, I interviewed Tony Blair for the Sunday Telegraph at Labour’s conference in Blackpool: mindful of the newspaper’s readership, he made an explicit pitch to “one-nation Tories” disaffected by John Major’s supposed “lurch to the right”. His message was: your true home now is New Labour and its “big tent”.

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Scroll forward 22 years: in Sunday’s Observer, Theresa May issued a mirror-image invitation to “voters who may previously have thought of themselves as Labour supporters to look at my government afresh. They will find a decent, moderate and patriotic programme that is worthy of their support.”

This claim was also at the heart of her speech in Birmingham on Wednesday. But her strategists evidently felt it was worth repeating the overture to centre-left voters disaffected by what she called the “Jeremy Corbyn party”.

Most senior politicians claim, on principle, to represent the “mainstream”, or a variant thereupon. It is too easily forgotten how much impact Ed Miliband made with his conference speech in 2012 declaring that Labour was now the true “one nation” party.

In Liverpool last month, Corbyn declared that his party represented “the new common sense” and “the new political mainstream”. At this stage, it is conventional for political columnists to mention something called the “Overton window”: this is not, in fact, a triumph of neo-Gothic stained glass of the sort that Alan Bennett might celebrate, or a prestigious box at a cricket ground. It refers to the range of ideas tolerated in public discourse – which, naturally, is always shifting in position and breadth.

Facebook Twitter Pinterest ‘In 1996, Tony Blair made an explicit pitch to ‘one-nation Tories’ disaffected by John Major’s supposed ‘lurch to the right’.’ Photograph: Tom Stoddart Archive/Getty Images

But the window that really matters to an incumbent or prospective prime minister is the pane of ideological glass that stands between them and the electorate. It is the frozen convictions of the party in question – and the leader’s job, in the politest possible way, is to chuck a brick through it, showing the voters that they are more interested in the interests of their fellow citizens than the creed of the members.

Tribes don’t win elections. Coalitions do. Notionally, this is now May’s official electoral strategy. She is presenting herself as the natural tribune of centre-ground voters disenchanted by Corbyn’s unabashed socialism, wondering, perhaps, if a new party will be established in the great wasteland between left and right.

Since the rich promise of her speech on “burning injustices” outside No 10 in 2016, May has tacked hard to the right with her continued insistence that “Brexit means Brexit” (whatever that means), her nativist attack on “citizens of nowhere”, and her remorseless emphasis upon immigration.

So her sudden swerve on Wednesday to claim that the Tories were, after all, the party of moderacy tested credibility. The flagship policy of the conference, after all, had been an immigration strategy that makes no economic sense but scratched the itch of the populist right. A similar instance of gesture politics was the announcement of a new stamp duty for foreign buyers.

Who would have thought, furthermore, that a Conservative foreign secretary, hitherto seen as a centrist, would identify parallels between the EU and the Soviet Union? And how seriously can we take the claims to middle-ground decency of a party whose conference stars were Jacob Rees-Mogg and the populist performance artist formerly known as “Boris”?

Would that it were not so: but modernisation is in sharp retreat, and has been for years. Back in the day, its two principal insights were as follows: first, that it was the Tory party itself, rather than Tory policy, that was the substantial problem. As long as Conservatives looked so homogeneous and sounded so unwelcoming, they were in strategic trouble.

Facebook Twitter Pinterest ‘Who would have thought that the Conservative foreign secretary, Jeremy Hunt, hitherto seen as a centrist, would identify parallels between the EU and the Soviet Union?’ Photograph: Christopher Furlong/Getty Images

Second, image management was not enough. There had to be substance to the claims of compassion and centrism. This is why the ringfencing of the NHS and international development budgets was so important to David Cameron and George Osborne; and why Cameron pushed through marriage equality in the face of strong opposition from his own party.

The most ridiculous claim made by some Tories around 2008-09 was that the decontamination process was now happily complete – rather like a driving test in decency, passed first time – and that normal service could be resumed. Worse, the dual pressures of the financial crash and, more recently, Brexit have crowded out the often contentious business of modernisation. It is much easier to shout about taking back control, slap lies on the side of buses and rage about immigration than it is to think deeply about the reasons for Corbyn’s comparative success in last year’s election and the fact that Labour’s position in the opinion polls has not been significantly affected by its torrid summer.

There are still admirable pockets of resistance in the party that, like Asterix the Gaul’s village, hold out against the populists and the nativists who have come to regard the 2016 referendum as something akin to a written constitution or holy mandate.

For now, they are on the losing side. The governing narrative of the party is not that a great persuasive campaign is required, but only a triumph of the will. This is the “lead” that Boris Johnson claimed to be putting in the party’s pencil in his own speech. It will not work, but it is so much easier than what is really required.

The problem is that May has expended all her capital trying to unite the Tories over Brexit. She has nothing left to force them to act like a party of the whole nation. Which is why her new, supposedly centrist position amounts to no more than words: a nice idea, an echo from another time, doomed to be lost like tears in rain.

• Matthew d’Ancona is a Guardian columnist