Press freedom was not invented in the United States, but there are few nations in which the importance of an independent press has been so closely woven into its long history. This great American tradition of civic respect for truth and truth-telling is now under threat. Donald Trump is not the first US president to attack the press or to feel unfairly treated by it. But he is the first who appears to have a calculated and consistent policy of undermining, delegitimising and even endangering the press’s work.

On Thursday, following an initiative by the Boston Globe, it is expected that some 350 editorial boards in news organisations across the United States will publish their own editorial comments on this issue. There is, of course, a risk in this initiative, and there will be differing press views about it. For some, including Mr Trump, it will feed the narrative that there is a partisan war between the press and the president. But the breadth of the response to the Boston Globe’s suggestion – and the fact that each editorial will be separately and independently written – suggests something different: that those who report and comment, day in and day out, in as professional and objective a manner as we can, are concerned that public respect for journalistic truth, reason and civility are under a new and present threat against which we must stand as best we can. As one editor has put it: we’re not at war with the Trump administration, we’re at work.

Mr Trump’s sweeping abuse of the press is grimly familiar now. He calls the US press “enemies of the people”. He accuses it – the instances now run into hundreds – of producing “fake news” and being “frankly disgusting”. He recently called the press “dangerous and sick”, and charged that they can “cause war”. He has called journalists “the lowest form of humanity”. The former FBI chief James Comey, later fired by Mr Trump, said the president asked him to consider jailing journalists for publishing leaks. His administration has removed reminders to respect press freedom from its internal manuals. He singles out four news organisations in particular – CNN, MSNBC, the New York Times and the Washington Post – insulting them in public (as he did at his Chequers press conference) and recently barring a CNN reporter from White House media events.

Play Video 1:23 The many times Donald Trump has attacked the media – video

Inescapably, this campaign has consequences. Public opinion, especially among Mr Trump’s most committed supporters, has become harsher in its views. Almost half of Republican voters (44%) believe the president should be able to close news organisations for “bad behaviour”, according to a recent poll. In another poll this week a majority of Republican voters (51%) say that the news media are “the enemy of the people”. The anti-media mood at some Trump rallies has been intimidating. Social media trolling, violent abuse and threats to journalists (especially sexual threats to women journalists) have reached unprecedented levels. The United Nations human rights commissioner warned this week that Mr Trump’s attacks on the press are “very close to incitement to violence”. In June, five staff members at the Capital Gazette in Maryland were shot dead by a local man with a local grievance. They may not be the last.

It is not the press’s job to save the United States from Mr Trump. It is the press’s job to report, delve, analyse and scrutinise as best it can and without fear. The press has many faults. It can be self-regarding. Far worse, in Mr Trump’s America, some parts of the media are partisan outlets which show a cavalier disregard for truth: the president has embraced these. But a free press must call out intimidation and incitement when it exists. And it must do what it can to preserve respect for the facts and for balanced judgment. In short, it must do its job. Mr Trump’s insults and incitements are a calculated danger to that, and to the respect, civility and dialogue that should exist between the press and its readers. The Guardian stands with the US press in its efforts to maintain the objectivity and the moral boundaries that this president – like so many others in much more dangerous parts of the world – is doing so much to destroy.