As Labour kept up its relentless focus on problems with the National Health Service, Johnson's Conservative Party tried to focus voters' minds on the prospect of an uncertain result and divided Parliament, which would endanger Johnson's plan to lead Britain out of the European Union on Jan 31.
Opinion polls give the Conservatives a lead over Labour, but all parties are nervous about the verdict of a volatile electorate that is weary after years of wrangling over Brexit.
Johnson's clumsy reaction to Jack's plight was a late misstep in a largely gaffe-free campaign. A video of the prime minister briefly declining to look at a cellphone photo of Jack on a journalist's phone — and then placing the phone in his pocket — has been viewed more than a million times.
In the clip of the interview, ITV reporter Joe Pike said to Johnson: "You refuse to look at the photo. You've taken my phone and put it in your pocket, prime minister."
Johnson then removed the phone from his pocket and looked at the screen.
“It's a terrible, terrible photo. And I apologize obviously to the families and all those who have terrible experiences in the NHS,” he said.
The incident quickly became caught up in a storm of social media claim, counterclaim and conspiracy.
Several prominent journalists, including the political editors of the BBC and ITV, tweeted a claim by anonymous Conservative officials that a party worker had been punched by a protester while Britain's health secretary visited the hospital.
When footage emerged showing that no assault had taken place, they apologized — but a media storm was already raging.
Some social media users circulated claims that the photo of Jack, first published by the Yorkshire Evening Post, was staged. Editor James Mitchinson tweeted his reply to one such reader, explaining the way the newspaper had verified the story.
“I would be happy to meet you over a coffee to offer you an explanation as to how sophisticated and corrosive the proliferation of fake news is, and what to do to guard against being conned by it," he wrote.
The controversy about the photo comes amid a campaign marred by misleading social media images and attack ads.
Britain's electoral laws, like those of most countries, were largely written before the dawn of the internet, meaning social media campaigns are mostly unregulated and open to exploitation. With no constraints, such as the strict rules that govern broadcasters, Britain's political parties have pushed the boundaries of truth, transparency and reality.
The Labour Party found itself embarrassed, meanwhile, by the leak of a phone recording to the right-wing political website Guido Fawkes in which the party's health spokesman suggested that the party would lose Thursday's vote because voters "can't stand Corbyn."
Jonathan Ashworth said his unguarded remarks were merely banter between old friends.
“Obviously with the benefit of hindsight I've been too clever by half and I look like an idiot as a result of doing it,'' he told the BBC later. "But I thought I was having a private conversation with someone who I've always had conversations with over the years.
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