The seismic political upheavals triggered by June’s calamitous vote to leave the European Union have greatly intensified in recent days, with the Tories at their conference in Birmingham following Labour and Ukip in a frantic, often divisive and persistently contradictory drive to identify and occupy the elusive “new centre ground” of British politics.

The most striking aspect of this struggle is that, in seeking to capitalise on the post-referendum state of flux, politicians in both main parties are paradoxically moving sharply away from the middle. In doing so, they set at risk fundamental liberal values and the universal, progressive principles that Britain, since the 18th-century Age of the Enlightenment, has been instrumental in spreading around the globe and in which its modern-day democracy and open society are rooted.

Theresa May’s first major speech as Conservative prime minister was intriguing for its renunciation of Thatcherism’s pernicious but still pervasive emphasis on individualism and antisocial self-interest. Taken by itself, this public recanting is as welcome as it is overdue. May stressed instead her intention to use the power of government to change and improve people’s lives, lauding “the good the state can do”.

But in anointing herself as standard-bearer for the interests of what she patronisingly called “ordinary people”, May tottered on the edge of the old Benthamite trap of suggesting she and her ministers know what is best for everyone. They should tread carefully. In a parliamentary democracy, overweening executive authority, convinced of its own moral rightness, is a creature to be feared, not admired, as America’s founding fathers knew full well when they built checks and balances into the US constitution.

Even as the Tories dream lazily of ruling in perpetuity, the rational parts of their brains must surely understand that May’s old school, top-down, take-what’s-good-for-you governance is neither desirable, democratic or effective. If May really believes they can become “the party of the workers, the party of public servants, the party of the NHS”, she must learn to talk to those workers, not down to them – and listen before leaping on issues such as grammar schools and Hinkley Point.

As a matter of respect and basic common sense, May should also heed the cautionary words of Thomas Paine, author of The Rights of Man, who wrote in 1792: “Mankind, as it appears to me, are always ripe enough to understand their true interest, provided it be presented clearly to their understanding, and that in a manner not to create suspicion by anything like self-design, nor offend by assuming too much. Where we would wish to reform we must not reproach.”

Worryingly, influential figures around May, pushing for a hard Brexit, are in danger of forgetting such wise maxims, assuming they were ever aware of them. What they call reform is unabashed regression and vengeful revisionism. There is a clear tendency, exhibited most brashly by Amber Rudd, the home secretary, and Liam Fox, the international trade secretary, to interpret the referendum result as unambiguous support for a divisive, rightwing agenda far removed from the political centre.

It is worth restating that a thumping 48% of those who voted, or 16.1 million people, chose to Remain, while 28% of those eligible to vote expressed no opinion either way. That result places those actively backing Leave in a distinct minority. They should show a little humility. Instead, ignoring Paine and the majority, they assume too much. They dangerously overreach.

Brexit did not, for example, provide a mandate for arbitrary restrictions on British-based companies hiring the best and brightest of foreign workers. In the integrated, globalised economy in which Fox claims a post-EU Britain will thrive, how shortsighted is that? Brexit was not a vote for British jobs for British workers, either, despite what Rudd insultingly implied. Many of the jobs in question are low skilled and badly paid; British-born workers simply do not want them. Some are demandingly high skilled, as in the NHS and care sectors, where there are critical shortages. Here, EU nationals perform vital work.

Nor was Brexit a vote to trash one of modern Britain’s great success stories – its internationally admired university sector – by limiting the numbers of foreign students who study here (and making it uncompetitively expensive for those who do). Already, the future of much joint research funding is in serious doubt. Notwithstanding vague government assurances about future financial support, Rudd’s reckless comments will only compound damaging uncertainty.

These and other noisome, ill-thought-out ideas, emanating from senior ministers bearing the seal of government, are as alarming as they are unpleasant. They carry a gross whiff of xenophobia. They convey an inescapable undertone of racism and intolerance. And they are a testament to what looks increasingly like an accelerating retreat from Britain’s liberal, inclusive and open-minded tradition and a return to the narrow, delusional world of Little England.

Despite much of what was said last week, Brexit was not a vote to scrap free access to the single market, thereby alienating Japanese and other overseas investors. Does Fox really think China, America or India will helpfully offer Britain, negotiating by itself, the same favourable terms that an EU bloc of 28 countries negotiating collectively has obtained? If he does, he should seek other work.

Do these hard Brexiters truly believe our European partners will sit on their hands if Britain sets about demolishing EU founding pillars such as freedom of movement and goods? Do they honestly pretend Britain can continue to trade while rejecting agreed EU rules policed by the European court of justice?

Do they somehow think British soldiers who commit war crimes should escape the sanction of European human rights conventions and international law? If they do, then dunderhead must be added to duplicity on the lengthening Brexiter charge sheet.

François Hollande, France’s president, is a socialist, but no radical. He faces a fierce challenge from the hard right in next year’s elections. Yet swiftly responding to the messages coming out of Birmingham, Hollande reacted sharply last week. Britain must and will pay if it persists in such gratuitous vandalism, Hollande said.

Viewed from the French, German and Italian mainstream, there is less and less to distinguish hard Brexiters from Marine Le Pen’s extremist Front National (which enthusiastically eggs them on) or the similarly xenophobic, nationalist Alternative for Germany. They are wreckers. They are reckless. They are irresponsible. They know only what they do not like. And they have little or no practical idea how they will replace the hard-won principles and institutions they traduce.

Whatever these delusional Tories believe, theirs is not the path to Britain’s new centre ground. Yet Labour, too, seems to have wholly lost its way as it struggles with internal divisions and the mortal threat to its working-class heartlands posed by Ukip.

Rarely has the country required a purposeful, effective opposition as badly as it does now. Rarely if ever has there been such urgent need of a champion ready to defend workers’ rights, regardless of race or nationality, to fight for the values of tolerance, inclusion and equality embodied in the European treaties and to lead those who feel threatened by mass migration, job insecurity, poverty and globalisation towards a broader understanding of who their real enemies are.

Stand up, Jeremy Corbyn, newly re-elected Labour leader. Except, on Europe, the defining issue of our time, Corbyn has consistently failed to stand up, show a lead or demonstrate an appreciation of the wider issues. Yes, more social housing is important. Yes, a higher minimum would be nice. Yes, the railways are a mess. But this is hardly the point.

Without a prosperous economy and expanding tax take, such policies will remain as unfunded wishful thinking. And if a hard Brexit takes hold, pushed through arbitrarily from next March onwards while a blinkered Corbyn vainly squabbles with his MPs and parliament is sidelined, much current public spending may be at risk, let alone any new programmes.

Corbyn must now demonstrate that he is the man to rise to this pre-eminently urgent national challenge. The line of attack is clear. It is not unpatriotic, as May & co perniciously suggest, to want this country to remain close to the EU and play as full a part as possible within Europe. It is not supercilious or elitist to suggest many Leave voters were egregiously misled about the consequences of Brexit by Nigel Farage, Boris Johnson and Michael Gove. It is only right to be concerned that this new administration is using Brexit as a means to foist a divisive rightwing agenda on the country. It is deeply worrying that, as a result, the Brexit process may be botched and our national interests, and our people, will consequently suffer across the board.

On one point, all perhaps can agree: this political crisis is about taking back control. But Brussels is not the enemy. The EU was always a straw man. Nor is the crisis about a loss of community, native culture and traditions, as some maintain. In all respects, these aspects of our national life are thriving, although they are evolving in ways some conservatives do not like.

No, this crisis is about reasserting and deepening democratic governance on behalf of all the people while limiting the control and excesses of those who, by one means or another, exercise power over us.

Just as in the time of Tom Paine and the American and French revolutions, it is a battle for equality, justice and tolerance, for the proud liberal principles of individual freedom, openness and inclusion. It is a struggle between the forces of reaction, prejudice, ignorance, dogmatism and self-interest and the universal vision of progressive societies in which the rights of all men and women are respected and advanced. It is an ongoing historical contest that everyone who cares about a fairer society must now steel themselves to fight again – and by all means, win. For here, not on Brexit’s wilder shores, is where Britain’s centre ground truly lies.