“Yes, what was that?” said Boris Johnson, affecting bafflement when asked about the Conservative party’s Twitter feed masquerading as a fact-checking service during the debate on Tuesday between the party leaders.

The prime minister appeared to be suggesting his own party’s official communications channels had nothing to do with him; he argued it was not for him to “invigilate absolutely everything” on the internet.

When pressed again to answer what the altered Tory website said about his campaign’s trustworthiness, Johnson simply launched into a diatribe about the need to check Jeremy Corbyn’s facts and pin down his position on leaving the EU.

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Deflect, evade, and change the conversation on to Brexit. These are the hallmarks of the prime minister’s strategy when it comes to answering difficult questions during the election.

The campaign so far has been marred by the dodgy editing of a video that made Labour’s Keir Starmer appear clueless, a personal letter from Johnson to voters in marginal seats selectively quoting a charity to make it appear supportive, and numerous statements examined by the Guardian’s fact-checking team that stretch the truth at best and lie at worst.

But his day on the stump in the north-east of England was dogged with questions about why people should believe him. It leaves questions about whether sharp practices by the Tory campaign are beginning to backfire for Johnson in the same way that Theresa May started to be haunted by accusations that she was too cowardly to do television debates with Corbyn.

“They need to be careful about trustworthiness. It’s a drip, drip, thing,” said one Tory strategist involved in the 2015 and 2017 elections, noting that negative perceptions of a leader can be cemented mid-campaign when they are being presented in a presidential style.

There is little sign that either the Conservative party strategists or ministers care about running a scrupulous campaign online. Insiders say an aggressive social media presence is part of the strategy this time, after the party felt comprehensively outgunned by Momentum in 2017.

“No one gives a toss” about the “social media cut and thrust”, Dominic Raab, the foreign secretary, said following the row over CCHQ’s Twitter name – changed to factcheckUK.

Play Video 1:00 Dominic Raab: Tory Twitter name change was 'instant rebuttal to Labour nonsense' - video

But despite his claims, the perception that Johnson’s campaign is cavalier with the truth may have cut through to voters and could make a difference.

Not many encounters between Johnson and the public have been recorded during the election because his campaign stops have been heavily stage managed, with limited access for journalists. The prime minister’s minders even banned a Daily Mirror reporter from attending a campaign tour of the north-east on Wednesday – something even the tightly controlled aides of Theresa May never did.

Johnson’s first campaign stop on Wednesday was a classic friendly scenario; he visited a washing machine factory owned by leading Vote Leave supporter, John Elliott, in the Labour-held seat of Sedgefield. He was pictured putting “Made in Britain” stickers on parts next to smiling staff members.

However, a second stop to address workers at a fabrication yard in Labour-held Stockton-on-Tees yielded a much more probing environment. The manager introduced Johnson as a “fantastic” man and the prime minister gave a stump speech focusing on getting Brexit done.

But a series of questions from switched-on employees tackled Johnson on difficult issues that went to the heart of trust – on why he had not released the report on Russian interference in the Brexit vote, and whether he could repeat his guarantees again that he would not sell off the NHS.

The question that then really wrong-footed the prime minister, prompting him to speak of a tax cut from the Tory manifesto, was from one Claire Cartlidge, a 35-year-old fuel quality manager. She challenged him over whether he really meant tax cuts would be for rich people or “people like us”.

Tory aides appeared to be taken aback by the announcement, which was meant to be a flagship promise released over the weekend, and pulled a broadcast interview so they could brief the prime minister on the details of the policy. But such was the climate of suspicion online that people immediately began to suggest it had been a deliberate ploy by the prime minister to deflect from the furore over the fake CCHQ Twitter feed.

As Johnson left the warehouse, a small group of protesters were audible outside of the perimeter fence shouting “liar, liar, liar”.

When he finally encountered some voters in an uncontrolled environment near a fish-and-chip shop on Saltburn sea front, the older crowd in a Tory-held seat were largely supportive and echoed back to him his phrase of “get Brexit done”. But a single heckling passerby shouted “liar” at him again, indicating it had become the insult of choice against the prime minister.