A moment from the 2016 campaign came back to me this week. Not the EU referendum, though that decision hovers over every aspect of this election, but rather the US presidential contest that same year. I was in Cleveland, Ohio, speaking to a proud member of Bikers4Trump, all in leather save for the stars and stripes bandana. What exactly was it about Donald Trump that appealed? “He’s honest,” came the reply.

I don’t think I concealed my incredulity. Honest? Are you sure? I rattled off the serial lies and proven falsehoods, a tally that, for Trump’s presidency, now approaches 14,000, at a rate of 22 false claims a day. But that didn’t faze the biker one bit. “He says what he believes,” he explained. While other politicians trimmed and euphemised, Trump just came out with it: insulting women, Mexicans, the Chinese, war heroes, you name it – political correctness be damned. What the fan on the Harley-Davidson had in mind was a different notion of honesty, one captured by the briefly fashionable apercu that Trump’s admirers took him seriously, even if they did not take him literally.

The memory of that resurfaced on Thursday night, as I sat in a small meeting room in a Holiday Inn in Newcastle-under-Lyme, just outside Stoke, watching a focus group made up almost entirely of Labour leave voters, convened by the public relations company Edelman. As they discussed the current campaign, what they believed and, more importantly, who they believed, it was tempting to conclude that we are living through Britain’s first post-truth election. Not in the sense of politicians lying – that’s not new – but in terms of the impact that post-truth, the casual disregard for the difference between truth and falsehood, is having on us, the electorate.

Early on in the session, the group was asked to write down the first word that popped into their heads when they thought of Boris Johnson. “Trump,” said one. “Liar,” said another. “Buffoon,” said a third. When asked to elaborate, a fourth said, “He talks well but you can’t believe anything he says,” citing the pledge to recruit 20,000 police officers.

‘The focus group might not have trusted the prime minister in general, but on Brexit they believed him.’ Boris Johnson by his campaign bus in Manchester. Photograph: WPA Pool/Getty Images

To which you might add Johnson’s promised 50,000 “new” nurses, a figure that includes 18,500 people who are already nurses. Or the 40 new hospitals that are really six upgraded hospitals. Or the £350m on the side of the bus. Or the fact that Johnson was sacked by both Michael Howard and the Times for lying. Or that he misled the Queen when he gave his reasons for proroguing parliament.

But guess what happened when, at the close, the scrupulously neutral moderator asked this group of past Labour voters who they would back on 12 December. All but one opted for Johnson. The same group that had declared him a liar nevertheless planned, quite cheerfully, to put him back into Downing Street. Why?

The core answer came back to Brexit and Johnson’s pledge to “Get Brexit done”. They might not trust the prime minister in general, but on Brexit they believe him – because, like Trump and his pledge to build a wall, they believe it’s what Johnson really thinks and what he really wants. When reminded of his broken die-in-a-ditch promise to leave by 31 October, there was a consensus around the table. “That wasn’t his fault,” said Tim, who’s semi-retired. “Parliament wouldn’t let him.”

The implication is that while the Westminster class, journalists and rival politicians, are focused on the literal truth – accurate stats, misleading claims – voters are looking for something different from a politician. Do you mean what you say and, crucially, will you make good on it?

Enter Jeremy Corbyn. The group were asked about him too and, in addition to calling him “indecisive”, “arrogant” and “weak”, three people offered that he too was a “liar” and “untrustworthy”. And yet while they forgave the dishonesty of Johnson, they gave no such leeway to Labour. The offer of free broadband was mocked, along with several other Labour manifesto promises. Jamie, who owns a car repair business, reckoned Labour had sat around asking themselves, “‘Who haven’t we given something to yet? I know, let’s do free dental care’. It’ll be free Pot Noodles for migrants next.” That brought laughter – and agreement.

‘While the focus group forgave the dishonesty of Boris Johnson, they gave no such leeway to Jeremy Corbyn.’ Photograph: Christopher Furlong/Getty Images

Make no mistake, these were not people who couldn’t do with the government spending more money. They all agreed that their town needed help and that things had got worse in the last 10 years (though, tellingly, none blamed the Tories for that, instead speaking of the decline as if it were the inevitable result of global, even natural, forces). Three of the women worked in local schools. And yet they dismissed Labour’s offer of new billions out of hand.

The objection was not that they, or their neighbours, don’t need the cash. Nor that they thought Corbyn wouldn’t want to spend it if he could. “I’m just not sure how achievable it’s going to be,” said Kath, a senior nursery nurse. She’s a Waspi woman; she’d love to have the pension money that is owed her. But the sheer size of the spending commitment made her distrust it. “Based on the past, all the things that were promised and not delivered,” nodded Zena. “Where’s the money going to come from?” asked Caroline.

There is a specific Labour dimension to this, a reputation for fiscal unreliability that dates back at least to the 1970s. But it also connects to a wider shift. Thanks in part to the cavalier disregard for truth that has come to characterise politics here and elsewhere, many voters now start from a base assumption that politicians are dishonest. Rows about details, numbers and figures especially, are shrugged off: “Who knows what’s true and what’s not? After all, they all lie.”

That helps a serial peddler of untruths like Johnson. But it also narrows the scope for what any politician can offer. They can, it seems, get away with making one big promise if that pledge seems both true to that politician and deliverable: “Here’s my oven-ready Brexit deal, let’s get it done,” fits that bill for Johnson (even if, of course, Brexit won’t be anywhere near done). But Labour’s long, expensive laundry list of costly spending promises falls at the second hurdle.

It’s a sour irony that Johnson, who did so much to shape this post-truth landscape, is currently benefiting from the scepticism it has nurtured – while it’s his opponents who are paying the price.

• Jonathan Freedland is a Guardian columnist