The first person to call Jeremy Corbyn “magic grandpa” was probably David Quantick, the comedy writer, in September last year. Obviously, insults are wrong and childish, but this was a clever one, almost a compliment, that summed up the surprising success of Corbyn’s TV persona in the 2017 election campaign.

The epithet was imported from the US, where it was applied to Bernie Sanders, the 76-year-old presidential hopeful. It was supposed to make fun of Corbyn’s unrealistic policies, but it also acknowledges the wizardry with which he fought the election.

One of the many ways in which we Blairites misjudged Corbyn was that we assumed that the more he was exposed to normal people who don’t pay much attention to politics, the less they would be likely to vote Labour. Even if people were uninterested in his past associations with the IRA and Palestinian terrorists, we thought his tetchiness and evasiveness when asked about them would count against him.

Instead, the Corbyn who asked the nation for its votes was a revelation. Despite the vast majority of his own MPs trying to get rid of him seven months earlier, and despite starting the campaign 20 points behind in the opinion polls, the Labour leader came across as relaxed, friendly and utterly reasonable.

Tough questions about his past associations were met with the amused detachment of a man of peace. For an electorate tired of falling real wages, rising housing costs and squeezed public spending on the NHS and schools, his promise of a modest relaxation of the public purse strings sounded like common sense.

This week, however, the tetchy and evasive Corbyn was back. Two days after he was pictured on the front page of the Daily Mail holding a wreath in Tunis, he was asked about it and said: “I was present when it was laid; I don’t think I was actually involved in it.”

Most voters have better things to do than to watch Sky News on a slow news day in August, but those words will echo for years – long after Corbyn’s sighs and the eye-rolling in response to further questions are forgotten.

If Corbyn is still Labour leader at the next election, no doubt he will resume his affable act for the duration of the campaign. He is good at it and he can pull it off, but the magic is not going to work so well the second time. The contrast between the terrorist-sympathising Marxist portrayed in the Tory press and the genial man of principle on the screen will have lost its power to surprise.

This week’s stories about Corbyn’s views on terrorism and antisemitism may not have much immediate effect on public opinion, but each one strips another layer of innocence from his image. Next time he would not be entering the campaign as the little known underdog, but as a leader who has been seven years in the public eye.

Corbyn’s team fought a brilliant campaign last time. The manifesto in particular was a work of art. They got away with promising a huge subsidy for the better-off (students) and forgot to promise to reverse cuts to tax credits. Next time, the media will treat Labour more as a possible government, and will ask more searching questions about spending promises and how they would be paid for.

The Jeremy Corbyn wreath-laying story Show all 3 1 /3 The Jeremy Corbyn wreath-laying story The Jeremy Corbyn wreath-laying story Atef Bseiso Facebook tribute The tribute to Atef Bseiso on Fatah's official Facebook page The Jeremy Corbyn wreath-laying story Corbyn wreath laying Labour leader Jeremy Corbyn attends the ceremony at the cemetery in Tunisia in 2014 where Atef Bseiso is buried Ambassade de Palestine en Tunisie/Facebook The Jeremy Corbyn wreath-laying story Benjamin Netanyahu Israeli prime minister Benjamin Israel has attacked Mr Corbyn over his attendance at the event Reuters

Nor is a simple anti-austerity message likely to be as successful. The significance of Theresa May’s decision to raise the rate of increase in NHS spending over the next few years has not yet sunk in.

Here is a simple test. Now that Jeremy Hunt is at the Foreign Office, who is the health secretary? It is interesting how few people can answer that question without googling. It could mean that, starting with a cleanish sheet, Matt Hancock (for it is he) could blunt some of Labour’s attack on public services.