Life is just too short, Siobhan McArdle said, to be a chief executive in the NHS right now. In an unusually blunt resignation letter earlier this month, the outgoing leader of the troubled South Tees hospitals foundation trust said her hospitals had delivered millions in so-called “efficiency savings”, but now the cupboard was bare. The trust had already been severely criticised by regulators for a bed shortage that was hurting patients; it couldn’t, she suggested, keep cutting without care suffering. It’s rare for a hospital boss to throw in the towel so publicly, but many in the NHS will understand where she was coming from.

The number of patients waiting for NHS surgery is now at its highest since records began in 2007. One in 11 NHS posts are lying vacant as staff quit and aren’t replaced. If we didn’t have a winter crisis last year, or not in the classic sense of patients piling up in corridors, that’s arguably because what was once an acute seasonal shortage of beds has become a chronic one rumbling all year long. For months all this has rather puzzlingly failed to get much political traction. But now, thanks to the father of a very sick baby girl, it has.

The angry father – Omar Salem – did the most dangerous thing a voter can do to a politician, which is to speak his mind

The video of a clearly emotional Omar Salem – buttonholing a chastened-looking prime minister in a hospital corridor about how there were “not enough doctors, not enough nurses” on the ward where his seven-day-old daughter was being treated at London’s Whipps Cross university hospital – has changed something. Yes, Salem is a Labour activist, a man with little time for Boris Johnson. But that doesn’t change the fact that he was right. A doctor on that ward promptly backed him up, telling the Guardian that the realities of life in a “chronically underfunded, understaffed and poorly resourced hospital” were covered up during Johnson’s visit. The only man twisting the facts here is the one filmed inexplicably insisting that there were “no press here” while standing beside a BBC camera crew. Salem merely did the most dangerous thing a voter can do to a politician, which is to speak his mind.

People ultimately decide what elections are about. It sounds obvious, but in the heat of a campaign – and Johnson’s recent flurry of walkabouts feel uncannily like the soft opening to one – it’s easy to forget who is actually running the show. Expensive strategists spend months war-gaming their campaigns, spin doctors build their grids of good news announcements, journalists endlessly second-guess what voters might be thinking. But all can be made to look ridiculous in a heartbeat by a passing member of the public spontaneously deciding to give a politician what for. In the end, people decide what they want to talk about, and it’s generally whatever is happening in their own lives.

Facebook Twitter Pinterest Gordon Brown speaking with Rochdale resident Gillian Duffy, in the run-up to the 2010 general election. Photograph: Jeff J Mitchell/Getty Images

Gordon Brown didn’t go to Rochdale to argue about migrants, but a chance encounter in the street with Gillian Duffy turned his campaign upside down. Tony Blair visited a Birmingham hospital to highlight Labour spending on the NHS, but was stopped in his tracks by Sharon Storer, angry about the cancer treatment her partner was getting. Theresa May held a snap election over Brexit, only to find voters had other ideas; the postmortem Downing Street conducted after the polls closed found that Brexit actually started sliding down the nation’s list of priorities the minute the election was called. With both main parties promising to take Britain out of the EU, people moved on to pondering what else they wanted from their next government.

This is the stuff of everyday lives, after all: your mum in tears because her operation was cancelled at the last minute; begging letters from the children’s school; the rough sleeper in the shop doorway. Had May’s team not worked so hard to avoid her meeting actual live human beings on the campaign trail, they might have realised this sooner.

It will be difficult to make the same mistake next time. In the space of a few days Johnson has been heckled by Britons living in Luxembourg, chastised by a woman in Doncaster market over austerity, and cornered in a hospital corridor. Tories who expected him to be mobbed by adoring fans wherever he went are discovering that Boris Johnson is electoral Marmite, someone people love or hate but rarely feel indifferent about.

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The stage is thus set for a bearpit of an election, as two polarising politicians – for Jeremy Corbyn evokes equally strong emotions – are forced into close contact with an unusually irritable public. Since a camera crew isn’t needed now to send any resulting encounter viral, just a curious bystander filming it on their phone, the surprise is arguably that it doesn’t happen more often.

Years of traipsing around behind politicians on carefully stage-managed visits taught me that the most interesting reaction often comes after they’ve gone. The circus that surrounds a visiting VIP – security guys muttering into earpieces, scrums of reporters, the general air of suppressed excitement – is oddly inhibiting. People feel they need to be on their best behaviour, or else suffer stage fright, thinking of what they wanted to say only after the politician moves on.

But the exceptions often are people who are so emotional they just can’t help themselves – and hospitals are places where love and fear and anger are perhaps unusually close to the surface. Storer was afraid of losing someone she loved. Salem’s baby daughter had, he said, nearly died. Johnson was right to say that these are exactly the sort of people any prime minister should be meeting. He is very wrong, however, if he imagines they can be fobbed off with yet more empty promises.

• Gaby Hinsliff is a Guardian columnist