Almost the whole chain of the us justice system is submerged in controversy of some kind. at the front end, there are the enforcers, the police, whose tactics have been the subject of protests since the killing of george floyd, a black man, in minnesota last may. at the remote extreme of the system, the supreme court is on the verge of a lasting shift to the right, to the anguish of liberal america. in between is the justice department, which donald trumps administration has often politicised to its own ends.

It is in this context that mr trump will announce his imminent nomination to the high court. whether he chooses amy coney barrett, barbara lagoa or another from his conservative shortlist, trust in american justice in large parts of the country is precarious. in a dread scenario, the new judge comes to rule on a contested presidential election after polling day in november. the president has already refused to commit to a peaceful transfer of power in the event of defeat to joe biden.

Mr trump will ignore the anxieties of his political enemies in making his choice. but the senate is not obliged to make it easy for him. given the stakes, this nomination demands special scrutiny from the upper chamber.

One basis for resistance is the timing. in 2016, senate majority leader mitch mcconnell blocked barack obamas nominee because, in an election year, with an outgoing president, the winner should have the choice. this invented rule was nonsense. but the unfairness would be compounded if, four years later, far closer to an election, the same man expedited a republican nominee. of the partys senators, at least a few (a potentially decisive number, given their slim majority) are known to have misgivings about a pre-election vote. if they assert themselves, mr mcconnell who on thursday pointedly pledged an orderly transition may yet favour delay. he still has until january before a new senate takes its seats.

Still, to dwell on process risks missing the larger picture. it is on the substance that the senate should ultimately push back against mr trumps nominee. classically, the chamber interrogates the hopefuls jurisprudence, their paper trail of decisions in lower courts, their instincts on such wedge issues as abortion, gun rights and, indeed, policing.

Senators might major this time on the judges attitude to executive power generally and to this president in particular. at stake is the new courts ability to constrain a sometimes rogue administration. there was some of this in the confirmation hearings for brett kavanaugh two years ago, but it was understandably overwhelmed by the sexual assault allegations against him. this time, it should be the dominant theme. that mr trump himself links the new justice to a post-election showdown over who governs is ominous.

No amount of questioning will bind his nominee once installed, of course. lifetime tenure liberates the justices from prior statements or outside pressures. but if the hearings expose signs of excessive deference to executive power, it could make some difference to what promises to be a close vote. and the us will know what it is in for.

If mr trump secures his third supreme court justice, he can lose the election and still go down as a vastly consequential president, shaping the law for a generation to come. all of this is within his rights. but the senate should not forget its own rights. the old convention that it nods through any qualified candidate is long gone. the obstructionism has often been cynical and partisan in nature. this time, there is a high-minded case for it.