With her commanding lead in the polls, all the talk was just how crushing May's defeat would be over hapless opposition leader Jeremy Corbyn.
So, as Britain embarked on delicate Brexit negotiations, May pitched the race as a choice between her "strong and stable" leadership and Corbyn's "coalition of chaos."
Now, with days to go until the vote, the picture is very different. The terrorist atrocities in Manchester
have transformed a humdrum campaign into one of the most extraordinary in recent times -- suspended twice out of respect for the victims, and now entirely reframed through the prism of security.
On the face of it, this should be solid ground for the Prime Minister: as Home Secretary, the job she held until last summer, she cast herself as an authoritarian, an image she reinforced on Sunday morning, in the wake of the latest attack, when she sent a warning to extremists that "enough is enough."
At a campaign event on Monday, she said voters had to decide "which leader does Britain trust to keep them safe."
Yet both May and Corbyn, the Labour Party leader, face tough questions about their records on security. May is vulnerable because, as Home Secretary for six years, she oversaw cuts to police officer numbers by over 20,000, affecting community policing -- which critics say has depleted the ability to spot extremism flourishing in neighborhoods.
During that time, May also watered down control orders, which imposed strict curfews on terror suspects, to a more flexible regime. Speaking at the site of the attack in Borough Market in central London, Britain's most senior police officer Cressida Dick referred to the concerns about police cuts when she said she would be looking for more resources to deal with the threat.
What's more, Britain's withdrawal from the European Union, under the terms of the hard Brexit that May has set out, is likely to see the country leave the pan-European intelligence-sharing organization Europol, which could further hamper counter-terrorism efforts.
Corbyn seized on May's vulnerability over police numbers on Monday, calling on her to resign. No matter that voters get the chance to kick her out on Thursday anyway, his call made headlines, and May was peppered with awkward questions from the media at her Monday campaign event.
Yet Corbyn has weaknesses on of his own: specifically in his past support for the Irish Republican Army, which perpetrated dozens of attacks on Britain, and Hamas.
He has also voted in the House of Commons against several counter-terror laws in the wake of 9/11 and the London bombings of July 7 2005.
He is also personally against Britain's independent nuclear deterrent, although if elected his party is committed to keeping it. The Labour leader has previously been equivocal on whether police should be able to shoot to kill terror suspects. In a sign that he wants to appeal to voters concerned about his record on security, Corbyn made clear his support for shoot-to-kill in a speech a day after the weekend attacks in London.
The key question now is whether the terror attacks will change people's minds on how they will vote on Thursday.
May's clear poll lead, which was as high as 20 points when she called the snap election, began to narrow after she unveiled controversial measures on the costs of elderly care.
Despite her claims to be "strong and stable", her poll rating did not recover after the Manchester attack two weeks ago, suggesting voters are not overwhelmingly convinced that she is the clear choice to "keep them safe". But neither has there been a clear surge for Labour, and Corbyn's personal ratings, while they have improved, still lag behind May's.
What is inescapable, is that an election that looked like a walk in the park for May when she called it nearly eight weeks ago now looks like a punishing slog. Whatever the result, May will surely be as happy as anyone when Friday morning arrives.