But now, after a day of voting, the future of her premiership, party and legacy could all be in doubt.
Many polls predicted a crushing defeat for Jeremy Corbyn's Labour, with the Conservatives gaining a considerable majority. Others were less generous towards May, and showed a dramatic narrowing between the two parties as their campaigns progressed. But even then, when the numbers were relatively favourable for Labour, the Conservatives were still out in front.
If you cast your mind back before the snap election was called, you may remember the double-digit leads the Tories held over Labour, and the huge advantage in popularity May had over Corbyn. With these in mind, many Labour supporters were resigned to the fact they were never going to win, and had their eyes on simply reducing Theresa May's majority.
But if the exit poll is accurate, this election could be one of the biggest political upsets in British history, beating even Brexit. The EU referendum that Leave won by 52 per cent was a huge upset, but was backed up by a number of polls – and predicted by some commentators – in the lead up to the vote. In comparison, almost no-one saw this election exit poll result coming.
So what is going on? Here is everything you need to know:
What just happened?
The shock final exit poll has predicted Theresa May will fall short of a ruling majority. The Tory Prime Minister currently has 331 seats in the House of Commons, and a majority of 17.
If this poll bears out, then this will be reduced to 314 seats, 12 short of of an overall majority.
Meanwhile, Labour could increase its number of seats by 34 to 266, while the Liberal Democrats are on track for 14. The SNP are predicted to suffer losses too.
Here are the full exit poll results:
Lib Dems: 14
Plaid Cymru: 3
Is the exit poll accurate? How did it fare in the last election?
Exit polls have always been close to predicting the winner, and with relative accuracy when it comes to seats. The only time this didn't happen was 1992, when it predicted a hung parliament, only for John Major to win a majority.
You can see below how the last election's exit result matched with the final results.
2015 Election exit poll vs final results
Conservative: 316 vs 331 (+15)
Labour: 239 vs 232 (-7)
SNP: 58 vs 56 (-2)
Lib Dems: 10 vs 8 (-2)
Others: 27 vs 23 (-4)
What will happen next?
If there is a hung parliament the consequences will be chaotic.
With Brexit negotiations just over a week away, it would cast huge uncertainty on how they may take place – especially if negotiations are still taking place on who will form the Government.
There is also the prospect of Theresa May resigning. The Prime Minister set out to win a significant majority in this election, and will have fallen dramatically short of her target if the exit poll is accurate. If she does stand down, this will trigger a tumultuous leadership battle.
The pound has already fallen by 2 per cent in response to the exit poll, and is certain to fall further if the exit poll bears out.
Will Jeremy Corbyn be Prime Minister?
At the moment it's impossible to say, and seems more unlikely than likely. But the fact we're even asking if Corbyn could soon walk into Number 10 is a stunning development in an election campaign where almost every expert assumed May would win.
The only way Corbyn would end up leading the country is if the Tories fail to win a majority and he is able to form a coalition Government with parties like the SNP, Lib Dems and Greens. But even then, he may still need to reach out to independent MPs and parties in Northern Ireland if he is to get enough seats to rule, and this could be incredibly tricky.
Early results also suggest Conservative numbers could have been underestimated by the exit poll. However, Labour is also reporting a significant increase in the youth vote in undeclared constituencies, which could end up skewering the Tories' chances of a majority.
Which is to say, everything is still to play for.
Will Theresa May resign?
If the Prime Minister doesn't increase her majority – or scores even more of an own goal with her snap election and finds it decreasing – then it seems very unlikely her position will be secure.
If she is forced to resign it will throw the UK's Brexit negotiations – which as mentioned already, start in just over a week – into absolute chaos.
Again, it's impossible to say. But if this exit poll is borne out, one thing is certain: it will not be good for Theresa May.
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