To be moderate is to be good. That is almost incontestable political wisdom. Nikki Haley, the US ambassador to the UN, who has declared her intention to step down, is lauded as the moderate in the Trump administration. Emmanuel Macron swept to power in France as the moderate keeping at bay the far right and far left. And when, last week, Theresa May made her appeal in these pages to Labour voters, she described her policies as “decent, moderate, patriotic”.

It’s not difficult to unpick such claims. Haley may not be as sulphurous as Donald Trump’s national security adviser, John Bolton, but from Iran to immigration she has given full backing to the presidential agenda. Macron’s moderate sheen has tarnished as he has come to be seen by many as the “president of the rich”. A prime minister who, as home secretary, inaugurated the “hostile environment” approach possesses little authority to give lectures on being decent.

Nevertheless, at a time when politics is increasingly polarised and the far right is on the march from Brazil to Sweden, to be moderate might seem a virtue. But is it?

To be moderate is good because it is not to be extreme and to be extreme, we know, is bad. May’s pitch is that she is neither Jeremy Corbyn nor Boris Johnson. “Moderate” here is defined more by what one is not rather than by what one is. And there is a large element of moral grandstanding: I am moderate because I am decent, unlike the wicked extremists.

Since what it is to be moderate is defined not in itself but in relation to the extremes, its meaning is constantly in flux. In the 1980s, Margaret Thatcher transformed the centre ground by demolishing the postwar Keynesian consensus. What was deemed moderate after the 80s was very different from what it had been in the 70s. To place the demands of profit over the needs of civic life, to support the marketisation of the social sphere from health to education, to view trade union power as destructive and seek to restrain it – much of this had seeped into the groundwater of the centre by the 00s.

Insofar as moderation is right, it is often right only after the event

Today, the rise of anti-immigrant populism has similarly reset what constitute moderate views on immigration. The creation of “fortress Europe”, the outsourcing of immigration policing to dictators and criminal gangs in north Africa and beyond, the criminalisation of rescue attempts – these are inhumane, immoral policies that once would have been regarded as intolerable. And yet most political parties, and much liberal opinion, now back them or, at best, keep silent about them.

Far from being a challenge to extremism, “moderation” is all too often an accommodation to it. At the Tory party conference, May announced the “end to austerity”. It was a meaningless declaration, but the fact that May felt compelled to make it exposes how opinion has shifted over the past decade.

In the wake of the 2008 financial crash, few argued against the need for austerity. The debate was largely about how quick should be the deficit reduction and how painful the cuts.

Today, it is seen as moderate to be critical of the impact of austerity policy and to demand its end. That should have been the default position all along. Except that, until recently, those who opposed austerity were dismissed as Marxists. Insofar as moderation is right, it is often right only after the event. It is non-moderates who help clear the path to more equitable ground.

Similarly with immigration. In time, we may come to see the EU’s current immigration policies as wretchedly heartless, just as we now regard austerity policies. It is possible that moderates of the future may declare an end to fortress Europe. If they do, they will have done so only because the case had already been made by those now dismissed as living on the political fringe.

I am not making an argument that one should be “extreme”. I am suggesting, rather, that the tag of being moderate is worthless. Its primary use is as a means of dismissing those who disagree with mainstream wisdom as extremist and their views as unworthy of consideration.

Yet on many issues, from abortion to free speech, from immigration to trade union rights, the reasonable, rational stance is to oppose what has come to be accepted as the moderate view. The argument, the policy and the consequences, not the label, are what matter.

• Kenan Malik is an Observer columnist