It is fair to say that when I argued at the London Press Club recently that journalists shouldn’t call Donald Trump a liar, it met with some customer resistance. That’s what he is! Why ever not? Call him out! You get the gist.

So perhaps I ought to explain further. I am not saying that the President tells the truth. On the day of his inauguration he made statements about the size of the audience that were demonstrably false. A few days later, he said that the only reason Hillary Clinton seemed to have more votes than him was because of the “millions of people who voted illegally”. This is about as certainly untrue as it is possible to be, and when Sean Spicer, the President’s spokesman, repeated it, it prompted The New York Times to run an unusual headline: “Press Secretary Affirms that Trump Believes Lie of Millions of Illegal Voters.”

I thought this was a mistake. I happen to think that any use of the L-word is a mistake, but I thought this was a specific error. I thought it undermined the NYT’s reputation as a news organisation. The trouble with the word lie is that it means, as the Oxford Dictionary puts it, “an intentionally false statement”. In the NYT headline it looks like a factual assertion, but it isn’t. It is not just saying the “millions of illegal voters” is a falsehood. By using the word lie, the NYT is calling Trump a liar. Which means it is saying, in a front-page news headline, that it has a window into Trump’s heart.

And yet the headline itself contradicts this interpretation. It says Trump “believes” the lie. In other words, he thinks it is true. So he was not lying: he believes someone else’s intentionally false statement. See the kind of trouble you get into?

The controversial orders Donald Trump has already issued Show all 9 1 /9 The controversial orders Donald Trump has already issued The controversial orders Donald Trump has already issued Trump and the media White House Press Secretary Sean Spicer takes questions during the daily press briefing Getty Images The controversial orders Donald Trump has already issued Trump and the Trans-Pacific Partnership Union leaders applaud US President Donald Trump for signing an executive order withdrawing the US from the Trans-Pacific Partnership negotiations during a meeting in the Roosevelt Room of the White House in Washington DC. Mr Trump issued a presidential memorandum in January announcing that the US would withdraw from the trade deal Getty The controversial orders Donald Trump has already issued Trump and the Mexico wall A US Border Patrol vehicle sits waiting for illegal immigrants at a fence opening near the US-Mexico border near McAllen, Texas. The number of incoming immigrants has surged ahead of the upcoming Presidential inauguration of Donald Trump, who has pledged to build a wall along the US-Mexico border. A signature campaign promise, Mr Trump outlined his intention to build a border wall on the US-Mexico border days after taking office Getty Images The controversial orders Donald Trump has already issued Trump and abortion US President Donald Trump signs an executive order as Chief of Staff Reince Priebus looks on in the Oval Office of the White House. Mr Trump reinstated a ban on American financial aide being granted to non-governmental organizations that provide abortion counseling, provide abortion referrals, or advocate for abortion access outside of the United States Getty Images The controversial orders Donald Trump has already issued Trump and the Dakota Access pipeline Opponents of the Keystone XL and Dakota Access pipelines hold a rally as they protest US President Donald Trump's executive orders advancing their construction, at Columbus Circle in New York. US President Donald Trump signed executive orders reviving the construction of two controversial oil pipelines, but said the projects would be subject to renegotiation Getty Images The controversial orders Donald Trump has already issued Trump and 'Obamacare' Nancy Pelosi who is the minority leader of the House of Representatives speaks beside House Democrats at an event to protect the Affordable Care Act in Los Angeles, California. US President Donald Trump's effort to make good on his campaign promise to repeal and replace the healthcare law failed when Republicans failed to get enough votes. Mr Trump has promised to revisit the matter Getty Images The controversial orders Donald Trump has already issued Donald Trump and 'sanctuary cities' US President Donald Trump signed an executive order in January threatening to pull funding for so-called "sanctuary cities" if they do not comply with federal immigration law AP The controversial orders Donald Trump has already issued Trump and the travel ban US President Donald Trump has attempted twice to restrict travel into the United States from several predominantly Muslim countries. The first attempt, in February, was met with swift opposition from protesters who flocked to airports around the country. That travel ban was later blocked by the Ninth Circuit Court of Appeals. The second ban was blocked by a federal judge a day before it was scheduled to be implemented in mid-March SANDY HUFFAKER/AFP/Getty Images The controversial orders Donald Trump has already issued Trump and climate change US President Donald Trump sought to dismantle several of his predecessor's actions on climate change in March. His order instructed the Environmental Protection Agency to reevaluate the Clean Power Plan, which would cap power plant emissions Shannon Stapleton/Reuters

The bigger trouble, though, is for the NYT’s reputation. Of course, it is a liberal newspaper. It endorsed Clinton. Its commentary is overwhelmingly hostile to Trump. But it is not a Political Action Committee or an anti-Trump clicktivist website. It is a giant of American journalism and a model for accurate, fact-checked reporting. Claiming to know the contents of Trump’s soul is not factual reporting.

But don’t we know that Trump knows that what he’s saying is untrue? We may not be able to see into his mind, but we do have the testimony of Tony Schwartz, ghost-writer of Trump’s memoir, The Art of the Deal: “He lied strategically. He had a complete lack of conscience about it.” If Schwartz can call him a liar, surely the rest of us can? I still think it is better not to.

Schwartz’s opinion is well founded. He got to know Trump, followed him around, wrote the book and then fell out with him. He now says that, as ghost-writer, he “put lipstick on a pig”. But it is still an opinion.

So that’s all right: we can call Trump a liar in comment articles? Of course we can, but I still think it is a bad idea. This is different from the point about news reporting. Accusing people of lying is poor journalism, but it is also poor politics. It is a bad way to make your case.

Remember Bob Dole in 1988. He told George HW Bush: “Stop lying about my record.” He came across as bitter and went on to lose. Remember Michael Howard in 2005. He unveiled a poster attacking Tony Blair: “If he’s prepared to lie to take us to war, he’s prepared to lie to win an election.” Howard then had to explain that he would have voted for military action in Iraq even if he had known there were no weapons of mass destruction, before going on to lose the election.

London protests against US President Donald Trump's travel ban

Blair himself took a different approach. “With each successive Tory leader, I would develop a line of attack, but I only did so after a lot of thought … The aim was to get the non-politician nodding … So I defined Major as weak; Hague as better at jokes than judgement; Howard as an opportunist; Cameron as a flip-flop.” (He didn’t bother with Duncan Smith: “The Tories did my work for me.”) He wrote in A Journey: “It’s not like calling your opponent a liar, or a fraud, or a villain or a hypocrite, but the middle-ground floating voter kind of shrugs their shoulders at those claims … they represent an insult, not an argument.”

If you want to make yourself feel better, call Trump – or Boris Johnson or Michael Gove – a liar. Other people who already agree with you might feel better, knowing that they belong to a like-minded tribe. But you will do nothing to persuade people who don’t like Trump much but who want America to be great again. Or people who have their doubts about Brexit but think the referendum result must stand. And there is a price to be paid in raising the emotional temperature of divisive politics.