At school I was told a cautionary tale about an art teacher who had the tips missing from two fingers. One had been lost showing a class how not to use a craft knife. The second was lost demonstrating what had happened the first time. The story may be apocryphal, but the lesson is real enough: there is no mistake so stupid that it can’t be made twice.

Boris Johnson still has time to bungle this election. If he fails to win an outright victory, his missing lead will take a special place in Westminster folklore alongside its spectral twin – the majority that Theresa May vaporised in 2017. That campaign, defined by the incumbent’s hubris, haunts the current one with unusual intensity, and not just because it is recent. It is hardly a memory; more a presence. It fuels Labour hopes and Tory fears. It lurks in every pundit’s caveat and every pollster’s preamble.

The prospect of expectations being upset, 2017-style, is used as a motivational tool by both Labour and Conservatives. For Labour it is a parable to prevent activists being disheartened by negative headlines. For Tories, it is a warning against complacency. It is deployed on the doorstep with lifelong Labour supporters who are impatient for Brexit, allergic to Jeremy Corbyn but reluctant to let go of an ancestral party allegiance. Tory candidates worry that those voters will presume that Johnson’s victory is in the bag; that enough other people are switching so they don’t have to.

The PM seeks to persuade undecided voters who see Corbyn as a crackpot, Labour as a party that likes to splash other people's cash

Projecting any campaign through the lens of what happened last time is bound to distort the picture. For one thing, behaviour can sometimes change in response to the expectation that it will stay the same. And a lot of stuff has happened since 2017.

Since the campaign began, I have been visiting constituencies, trailing candidates and phoning activists to get some sense (away from opinion polls) of what is going on. The reports contain shades of familiarity from the last race, but also vital differences. With the obvious proviso that my accrued anecdotes are unscientific, I detect two significant shifts.

First, suspicion of Corbyn has crystallised into something harder for Labour to crack. His candidacy was once seen as eccentric, with the redeeming qualities of mildness and candour. Now canvassers encounter a more visceral aversion to the Labour leader. Some people cite specific objections to Corbyn’s policy positions and record of past association with extremists, but for others it is a generalised cultural recoil. The word “dangerous” comes up a lot, as does “weak”.

A second change is that leavers did not think Brexit was in serious peril at the last election. May looked set to do the business. Besides, Corbyn pledged to honour the referendum mandate. Now he wants a second vote, and seems shifty on the whole subject. In 2017, pro-Brexit voters who hated the idea of switching to the Tories could find reasons to stick with Labour. This time they might not take any chances.

Those factors explain Johnson’s campaign strategy in its entirety. The Tory manifesto is deliberately flimsy. Arming the prime minister with fewer pledges limits his ability to discharge the weapon into his own feet. May was hobbled by her own ill-targeted policies in 2017. But, more important, a forgettable manifesto means the campaign can move swiftly back to what Tory strategists see as their core strengths: Brexit (getting it done) and the choice of prime minister (not Corbyn).

The most cunning inflection in Johnson’s rhetoric is his portrayal of EU withdrawal as something mundane, “oven ready”, “just add water”. That plays to leavers’ bafflement at the delay and their suspicion that claims of technical complexity and economic hazard are just excuses; that really it is a story of self-serving politicians refusing to do the one perfectly simple thing that a majority told them to do.

It is dishonest to depict Brexit as a chore when it is closer to a revolution, but that frame artfully positions the Tories as the less extreme option on the ballot paper. Committed remainers won’t be persuaded, but they aren’t the target audience. Johnson is speaking to undecided voters who are predisposed to see Corbyn as a crackpot, Labour as a party that loves splashing other people’s cash, and socialism as something that went wrong in the last century.

Facebook Twitter Pinterest Theresa May squandered a perfectly good majority and paid the price. What have the voters in store for her successor? Photograph: Tolga Akmen/AFP via Getty Images

The good news for Labour is that Johnson’s personal brand clashes with a campaign built on trust and getting stuff done. His record bears no scrutiny on either trait. His public performances are not as brittle and alienating as May’s were, but nor does he sound vastly more professional than Corbyn. Conservatives speak of a fabled “Heineken effect” – a charismatic connection that allows Boris to reach otherwise unavailable parts of the electorate. I have yet to witness it. I have, by contrast, heard concern even among Tory voters that the prime minister is unserious and a stranger to honesty.

Johnson’s appeal is weaker with women, and falls away among younger voters. If the latter account for the late surges that have been reported in electoral registration, the demographics of the race could be markedly altered in Labour’s favour. All sides are expecting the race to tighten. Smaller parties are being squeezed. Tory candidates are braced for a shock, straining to hear the echoes of 2017 so as not to be surprised if it happens again.

At some level they seem to know that they are pushing their luck, asking for yet another term of unloved Tory rule, under a famously fallible prime minister. They know that the window of opportunity is small and closing – that Brexit fatigue and dread of Corbyn are carrying them towards the finish line, while an election on other issues against almost any other Labour leader is one they might already have lost.

You see it written on Johnson’s face too, at unguarded moments: the flicker of a guilty smile, the glint of disbelief around the eyes that, yes, he really is getting away with this; the furtive shuffle, like a schoolboy shoplifter, with unearned electoral advantage stuffed down his trousers, sidling past the checkout hoping not to trigger any alarms before polling day.

• Rafael Behr is a Guardian columnist