The standard way of describing a move such as the one Theresa May made on Tuesday morning is to call it a “gamble”. A prime minister with a Commons majority and three years left to run on her parliamentary term does not throw that away without risk. In that sense, May has gambled – but as gambles go, it’s about the surest bet any politician could ever place.

The polls show May and the Conservatives stretching so far ahead of their opponents that their lead over Labour is about the same size as Labour’s total vote. One survey on Monday showed Labour at 23%, with the Tories on 44%. At that rate, Labour could be on course for a performance worse than their trouncing in 1983. Some have been dusting off the record books to look at 1935, when Labour won just 154 seats.

General election 2017: Theresa May says opposition to Brexit plan behind decision - Politics live Read more

The polling guru John Curtice may well be right that that lead could narrow in the heat of a campaign – especially one that, at 50 days, is relatively long by British standards. There is also a risk that calling a snap vote will irritate an electorate weary of being summoned to the polling booths so often. But even if the Conservative lead shrinks by five or even 10 points May will still win, and win big.

Given all that, the real gamble for May would have been to pass up this chance and sit it out until 2020. When asked who would make the best prime minister, 50% of Britons name May, compared with just 14% who pick Jeremy Corbyn. As May spent the Easter holiday hiking in Snowdonia with her husband, the temptation must have gnawed away at her. When again would the odds be so much in her favour?

And bear in mind that until now the Tories have barely had to lay a glove on Corbyn: they’ve left that job to the Labour leader’s internal opponents. But Conservative strategists all but slaver at the juiciness of the prey they will now hunt. I’ve heard Tories speak with delight at the prospect of reminding voters that, for example, Corbyn was prepared to see a marriage break up rather than sanction his child going to a selective school. They think such titbits mark Corbyn out as a “weirdo”, utterly out of step with mainstream, aspirational Britons. Think what the Tories and their press allies did to Ed Miliband, Neil Kinnock and Michael Foot. Then consider how they will feast on Corbyn.

So this was a gamble for those who like their wagers ultra-low-risk. The greatest downside for May is in undermining her brand as a political straight-shooter, one who eschews the playing of Westminster games. In her Downing Street statement she nodded to that, speaking of her “reluctance” in breaking her earlier pledge to call no snap election. But she will have calculated that the benefits heavily outweigh that cost.

Play Video 1:04 May's election U-turn: the times she ruled out a snap vote - video

And what are those benefits? The most obvious relate to leaving the EU. Brexit will now have a double mandate. Not content with the referendum of 2016, May has decided to buttress that verdict with a general election victory. She will claim the anticipated win on 8 June as an endorsement of her Brexit strategy. Remainers will have to face the fact that quitting the EU will have been approved by the British people twice in a single year. As May put it: “Britain is leaving the European Union and there can be no turning back.”

The advantages for her are obvious. That enhanced and personal mandate will give her extra muscle in negotiations with the remaining 27 EU states. True, it will also cut short the already absurdly brief time span for those talks. But it does mean she will have more leeway than she might otherwise have enjoyed. If she is forced to accept, say, a hefty divorce bill from the EU, she can do it without worrying about paying an imminent electoral price. On 8 June she expects to receive a large chunk of political capital, which she can spend over the next two years.

Labour is in deep trouble, but it’s our only defence against a Tory landslide | Owen Jones Read more

Better still for her, she will have broken the link between the article 50 and UK electoral timetables. Previously, she faced a two-year talks deadline, which would expire in 2019, followed by a general election a year later. Now she has ensured that day of judgment from the electorate is postponed to 2022. She has bought herself crucial time.

At home, too, this helps. All prime ministers yearn for their own mandate: it weakened Gordon Brown not to have one. Now May will have hers. If she wins a serious majority, she will reduce the leverage her backbench ultras currently hold over her (that too will give her more leeway in the Brexit talks).

And she’ll be free of the 2015 manifesto of David Cameron and George Osborne. Increasingly, May and her ministers were treating that document as if it were a historical artefact, one that no longer applied. But it was awkward – as the chancellor, Philip Hammond, found when he had to drop his proposed change to national insurance on the self-employed because he had broken a promise in a manifesto he manifestly no longer believed in. Now May will be able to set out her own stall, knowing what most gamblers can never know – that she is almost certain to win.