Brendan Behan described critics as being like eunuchs in a harem. “They know how it’s done, they’ve seen it done every day, but they’re unable to do it themselves.” If liberals are not careful, the same will be said of them. They know how Trump, Brexit and Orbán won. They see how the world has gone wrong every day. But they are unable to say what they’d do about it.

Take that model modern liberal, Harvard University’s much-televised philosopher Michael J Sandel, who delivered his contribution to the populist debate last week. Those who must fight Trump or Brexit listen to him and so many of his contemporaries, and leave with one question preying on our minds: “Yes and –.”

In Sandel’s case, the first warning sign comes when he accepts the radical right backlash sprang from “legitimate grievances”. Rather too conveniently, the grievances he accepts as legitimate are complaints the liberal-left find congenial: inequality, stagnant wages, the soulless technocratic politics of the Clinton/Blair years. Who among us will object when Sandel says: “Like the triumph of Brexit in the UK, the election of Trump was an angry verdict on decades of rising inequality and a version of globalisation that benefits those at the top but leaves ordinary people feeling disempowered.”

His emphasis on the economic anger of the white working class is forgivable but misses how many among the wealthy endorsed Trump and Brexit. And of course, like diners swerving to avoid tramps in restaurant doorways, it daintily sidesteps awkward questions about race. Given these handicaps, his programme for action is vague to the point of vacuity.

Michael Sandel: “The grievances he accepts as legitimate are complaints the liberal-left find congenial.” Photograph: Christopher Thomond/The Guardian

Liberals must tackle inequalities of power and wealth, Sandel says. But how? They must defend the dignity of labour and stop humiliating workers whose jobs are being destroyed by modern technology. Does that mean we should reject proposals for a basic income being put forward by a suspiciously chummy alliance of Silicon Valley billionaires and leftist utopians? Sandel raises the question but does not answer it. Liberals must understand patriotism and accept that a majority believes we owe more to our fellow citizens than citizens of other countries. Just so. But what then should be done about immigration and terrorism, concerns that powered the right’s rise? Answer comes there none.

It is not often I sympathise with politicians. But the leaders of the opposition to the new status quo must read liberal commentary and wonder what the hell they are meant to do with it.

US liberals can dodge awkward questions. They can hope the Mueller inquiry will lead to Trump’s impeachment, and spare them the need to explain why so many voters turned on them.

Their British counterparts cannot. They need practical strategies urgently. Instead, they have a Labour leadership that wants to take Britain not only out of the EU but the single market. Jeremy Corbyn and John McDonnell are following the old Bennite fantasy that the left can build socialism in one country once Britain is alone. The sheer stupidity of what passes for their thinking cannot be overstated. Nine of the 11 most equal countries in the world are in the EU and the 10th, Norway, is in the single market. Inequality in Britain soared because of decisions taken by the Thatcher administration 30 years ago. Those who remember the Thatcher years will recall Europe was just about the only thing that kept British trade unionism viable. Unless the Labour left wants full communism – and if it does, it should say so – there is nothing in the single market rules to prevent it from building a more social democratic society.

Sandel’s explaining away of the modern right by imagining its supporters share the economic grievances of the left may be too self-serving to take seriously. But it’s not a delusion the Labour leadership can indulge. Labour cannot say it wants to address the grievances of the “left behind” and end austerity when leaving the single market will leave the poorest regions of Britain even further behind and make financing a leftwing programme impossible.

Then there is the question of race, that commentators can dodge but the rest of us cannot. The right has triumphed by playing on white identity politics; explicitly so in the case of Trump, Orbán and the Brexit campaign. In other words, the worst of the right has aped the worst of the left. A successful political strategy would combine the fight against white racism with a contemptuous dismissal of liberal-leftists who engaged in racial sectarianism and the separation of ethnicities. It would ask them to think about the possibility that cries of “white privilege” drive away poor whites, who are anything but privileged, and who might once have given the left a hearing.

Fighting culture wars is easy. If social and traditional media are a guide, modern people want to fight little else. But in Britain’s case, those of us engaged in the harder task of trying to stop a hard Brexit have to say what they would do about immigration. Staying in the single market might help save us from another decade of economic stagnation, but the cost of admission is accepting freedom of movement. You could argue that freedom of movement for Europeans would be balanced by tight restrictions on migrants from the rest of the world, although honesty would require you to admit that most of those immigrants are needed. Or you could defend the pre-2017 immigration system. You cannot avoid the issue, however. You must have an answer to the question “What would you do instead?”

The British government looks close to disintegration. The pressure on Corbyn to oppose rather than cover for it will grow. Even talk of a second referendum sounds less far-fetched than it did. But nothing will be won in Britain or America unless Trump and Brexit’s opponents show more intellectual rigour than Harvard professors can muster.

• Nick Cohen is an Observer columnist