If Donald Trump should win the presidency it would represent a threat to press freedom, says the Committee to Protect Journalists (CPJ).

The US-based press freedom watchdog has taken what it calls “an unprecedented step” by issuing a statement by the chair of its board, Sandra Mims Rowe, in which she accuses Trump of consistently betraying first amendment values.

Earlier this month, said the statement, CPJ’s board of directors passed a resolution declaring Trump to be a “threat to the rights of journalists and to CPJ’s ability to advocate for press freedom around the world”. It continued:

Since the beginning of his candidacy, Trump has insulted and vilified the press and has made his opposition to the media a centrepiece of his campaign. Trump has routinely labelled the press as ‘dishonest’ and ‘scum’ and singled out individual news organisations and journalists.

It recalled that Trump, the Republican candidate, “mocked a disabled New York Times journalist and called an ABC News reporter a ‘sleaze’ in a press conference”.

It also stated that Trump had refused to condemn attacks on journalists by his supporters while his campaign team had “systematically denied press credentials to outlets that have covered him critically”.

Rowe’s statement mentioned the rally in Texas in February, in which Trump pledged to “open up our libel laws so when [newspapers] write purposely negative stories… we can sue them and make lots of money”.

In September, Trump tweeted: “My lawyers want to sue the failing @nytimes so badly for irresponsible intent. I said no (for now), but they are watching”.

The CPJ statement said: “While some have suggested that these statements are rhetorical, we take Trump at his word. His intent and his disregard for the constitutional free press principle are clear”.

It believed a Trump presidency would have a negative effect on press freedom outside the United States, arguing that any failure by the US to uphold its own standards would embolden despotic leaders to restrict the media in their own countries. The statement concluded:

Through his words and actions, Trump has consistently demonstrated a contempt for the role of the press beyond offering publicity to him and advancing his interests... This is not about picking sides in an election. This is recognising that a Trump presidency represents a threat to press freedom unknown in modern history.

The statement was issued before Trump implied that the New York Times was part of a Mexican conspiracy to undermine him.

This was the “logic” of his claim: “The largest shareholder in the Times is Carlos Slim [who]... comes from Mexico. He’s given many millions of dollars to the Clintons and their initiative.”

So, in Trump’s view, NY Times reporters are “not journalists, they’re corporate lobbyists for Carlos Slim and for Hillary Clinton”.

Trump returned to that theme on Sunday in a tweet saying: “The election is absolutely being rigged by the dishonest and distorted media pushing Crooked Hillary...”

Trump’s claim has been supported by Newt Gingrich, a former Republican House of Representatives speaker. During a Fox News interview he said: “Fourteen million people picked Donald Trump. Twenty TV executives decided to destroy him.

“Without the unending, one-sided assault of the news media, Trump would be beating Hillary Clinton by 15 points”.