When Alistair Cooke was still alive, one of the first obligations of any recently arrived Guardian correspondent in the United States was to pay a courtesy call on the Guardian's legendary American doyen. Cooke would start by making three pronouncements. Understand that the United States is a foreign country. Travel regularly outside Washington. And make sure you grasp the absolute importance of the US supreme court.

No one arriving in Washington this week could fail to grasp the enduring potency of the last of these three pieces of advice. This week's supreme court hearings into the constitutionality of Barack Obama's healthcare legislation are not something on the arcane margins. Instead they have all the hallmarks of an event that has the potential to set the nine-judge court fundamentally at odds with Congress, the president, and even with the very idea of effective federal government.

The outcome of this week's hearings will come in June, when the court delivers its verdict. But the way the case has been choreographed makes clear that the court grasps the scale of what it has embarked on. The fact that it has unusually set aside three whole days this week to allow different aspects of the healthcare system to be argued is a sign that this is no run of the mill hearing.

Which indeed it is not. There are three main reasons why the case of Florida et al v US Department of Health and Human Services et al matters so much – and not just to Americans. One is the sheer scale of the US health industry, which takes up more than a sixth of the entire US economy but does not cover, broadly speaking, the poorest third of the population. The second is that the massively expensive, massively inadequate system has defied all efforts at serious reform for generations – including very nearly defying those of Obama, even when he had a Democratic majority on Capitol Hill.

But the third is that the current supreme court is something new. By appointing two extremely conservative justices to the court, George Bush set in train a reactionary judicial counter-revolution which, as the liberal jurist Ronald Dworkin has put it, is "Jacobin in its disdain for tradition and precedent." Since joining the court, Bush's picks, Chief Justice John Roberts and Justice Samuel Alito, have put rocket fuel into the conservative constitutional project that was already being promoted by long-serving Justices Antonin Scalia and Clarence Thomas. As long as they can capture the vote of the man in the middle, Justice Anthony Kennedy, the judicial gang of four seem set on using the court to polarise American life the way the Tea Party tries to do in traditional politics.

This palpable shift has been apparent in several decisions, in almost all of which the conservative majority has carried the day by five votes to four. These have included rulings that stop cities like Seattle and Louisville arranging their school intakes to prevent racial ghettos, or that undermine longstanding anti-trust laws preventing price-fixing. But the two cases which have had greatest publicity – until now – have been a decision to throw out all restraints on the amount that corporations can spend on political campaigning (a decision which has allowed Mitt Romney to dominate the Republican presidential contest) and a ruling which struck down a District of Columbia law preventing the ownership of handguns.

In legal terms, these cases range widely across the field. What increasingly unites them, however, is not so much a coherent conservative doctrine of law like so-called originalism, which sees the original constitution – which upheld slavery – as an almost sacred document that it is the job of the justices to maintain. More dominant in some recent cases has been a much more politically consistent sense of what it means to be an American conservative in fields from religion to race to commerce and government – the ultimate example of this approach being the Bush v Gore ruling in 2000. This approach was expressed by the normally taciturn Justice Clarence Thomas, who told a university audience in 2010: "If it's wrong, it's wrong, and we are obligated to revisit it."

The fingerprints of this approach have been all over the healthcare case. Rarely has any legal challenge to the federal government been so explicitly partisan or co-ordinated. But the court has seemed eager to get involved too, fast-tracking the case to this week's hearing. Remarkably, Justice Thomas also continues to sit in the case, even though his wife Virginia is an active paid lobbyist for an organisation, Liberty Central, which campaigns against the healthcare act. But these are new times not just in American politics but in American law.

The politicisation of the courts has deep roots. Today's conservative judicial aggression is in some ways merely a modern acceleration of processes which Alistair Cooke would have recognised from the 1930s, when the supreme court fought against Roosevelt's New Deal. But it is also a lesson in how the politicisation of the law can get increasingly out of hand in an increasingly polarised nation and dysfunctional polity. If the court sweeps away healthcare – as the reporting of this week's hearings strongly implies that it will – the five judges will not just have entered the political arena but laid claim to control of it.

All this is a world away from the role of British – and most European – judges, and of sensible pragmatic American judges like Justice Stephen Breyer, in his important recent book. Yet we are beginning, even here, to drift into a more explicitly politicised set of interconnections between politics and the courts. Judicial review is as widespread here. Our judges make more speeches than they used to. This week, a Northern Ireland judge has decided to issue contempt of court proceedings against the former cabinet minister Peter Hain. Yesterday a House of Lords report pressed for changes in the way the judges are appointed to ensure greater diversity.

The government remains committed to enacting a British bill of rights in place of the Human Rights Act, and is regularly picking fights with the European courts.

America's dystopic politicisation of the courts and the judiciary belongs, as Alistair Cooke said, to a foreign country. But this is a slippery and dangerous slope down which we should not allow ourselves to slide.

• The original text of this article stated that 'a Northern Ireland judge has decided to sue the former cabinet minister Peter Hain'. This statement was erroneous as the court had in fact issued contempt of court proceedings against Mr Hain. This has now been amended. Thanks to commenters for pointing out this error