On paper, Theresa May can’t lose – but no election is risk free | Robert Ford

After the Brexit referendum, the Brexit election. Less than a year after the historic vote to end Britain’s membership of the EU, and less than two years after her party secured an unexpected parliamentary majority, Theresa May is seeking another mandate from the country – to deliver on the fundamental change demanded a year ago. The announcement on Tuesday caught many unawares, but the logic of the decision is clear.

The Conservatives’ recent poll leads over Labour have been topping 20 points: landslide territory. After 2015, headline poll numbers are treated with a bit more caution, but there is nothing but good news for the Conservatives in the polling fine print, too. May enjoys immensely strong personal ratings, while Labour leader Jeremy Corbyn’s continue to be among the worst ever recorded by an opposition leader. When voters are offered a straight choice between May and Corbyn, the Labour leader comes a distant third – well behind “don’t know”.

The Conservatives lead on almost every issue, and with almost every demographic group. In some polls, May is preferred to Corbyn among Labour voters. The question even the most cautious Conservative strategist must ask themselves when pondering such data is: “If not now, when?”

But there is one reason for caution: while all the stars seem aligned for the Conservatives, no one knows for certain how the new political divides opened up in last year’s polarising referendum will affect voters’ behaviour.

The prime minister’s gamble makes sense given the prizes on offer. May has the chance to lead the largest Tory majority since the 1980s, and set up a decade or more of political dominance. A large majority would free her to undertake the complex Brexit negotiations as she sees fit, with a secure majority for whatever deal she announces. An influx of new MPs elected on the coat-tails of a popular prime minister should be more loyal to her personally. This in turn will enable her to marginalise, or ignore, hardcore Eurosceptic serial rebels and disaffected Cameron and Osborne allies.

An election victory will give May a fresh mandate for her own domestic political agenda, freed from the constraints of the 2015 manifesto, and the personal authority to take the country in a new direction. Going early also enables May to seize the fleeting opportunity to recruit Ukip voters while they are briefly happy with the government’s agenda on Brexit and immigration, and while Ukip is (even by its own standards) unusually weak and divided. The habitually distrustful and politically disaffected Ukip supporters are unlikely to remain content for long, so it makes sense for May to bank their votes while she can, before the tough process of negotiation with the EU gets under way.

While May’s starting position is as strong as any prime minister’s since Tony Blair in 2001, there is no such thing as a risk-free election. The most alarmed MPs on the Conservative benches will be the large cohort elected in 2015 at the Liberal Democrats’ expense. There have already been leaked reports of polling showing that many face a tough rematch, and they will now have to enter it without the benefit of a full parliamentary term to build local profile and a personal vote. The small gap between elections, and the apparent recovery in party fortunes, will also encourage many defeated Liberal Democrat MPs to stand again, presenting local voters with a familiar and often well-liked alternative to their incumbent. The party may also be helped by local “progressive alliance” arrangements with lower-placed Green or Labour candidates agreeing to limit their campaigning to assist the best-placed opponent to the Conservatives. On the other hand, the sheer scale of the Lib Dems’ 2015 defeat will give comfort to many nervous MPs who have the cushion of a large majority that should be further padded by the recent upward trend in Conservative poll numbers.

While Labour looks weaker on every polling metric than it has been for a generation or more, there are still things for Conservative strategists to worry about. Labour has enjoyed a huge surge in membership since 2015, providing a major new resource to help turn out lower-interest voters, or persuade wavering traditional supporters back into the fold.

Corbyn’s weakness may paradoxically prove useful to the broader party – voters who like the party or their local MP, but dislike the party’s leader, can be reassured that he has little chance of governing. Labour can also frame the election as a last chance to prevent a landslide Conservative government, liberated to pursue a full-blooded rightwing agenda for the first time in a generation.

Turnout may be low, as voters tired of repeated elections tune out of a contest whose result already looks all but certain, with unpredictable results. This may also play in Labour’s favour if voters in heartland seats, torn between their historical loyalty to Labour and current preference for Brexit and May, resolve these internal tensions by sitting out this contest. Alternatively, it could hurt Labour if energised Conservative Brexiters turn out in droves while Labour’s more lukewarm supporters stay home.

The worst case scenario for May would be a marginal increase in her majority, but a major increase in the difficulty of managing it, as modest gains from Labour, achieved mainly by eating into the Ukip vote, are mostly offset by losses to the Liberal Democrats, achieved in part by recruiting Conservative Remain voters. May would then face trouble on both wings of her party.

Hardline Eurosceptics would be reinforced by new MPs aware that they owe their victory to Ukip votes, while Conservatives who have just survived close races against resurgent Liberal Democrats would be more reluctant to endorse a hardline approach on Brexit. May’s authority would also be eroded, as such a result would fall well short of her party’s sky-high expectations.

While the Conservatives begin as strong favourites, the scale of their victory remains highly uncertain, and dependent on a complicated and fragmented political competition. There are two ways to think about the electoral battlefield. The traditional, and simpler, way is to focus on the overall swing between the two parties. The current polling average has the Conservatives around 45% and Labour around 25% – a 6.5-point swing from Labour to the Conservatives since 2015. A swing this size could deliver 60 Labour seats to the Tories, most likely resulting in a three-figure majority. However, even a relatively modest decline in this gap could substantially cut into these gains, because there are fewer traditional Labour-Conservative marginal seats than in any election since at least 1955. A swing of around five points delivered 87 Labour seats in 2010; a similar swing this year would yield around 40. The concentration of both parties’ votes in very safe seats reduces the chances of a Conservative landslide or a Labour wipeout.

The two-party swing is, however, only part of the story in a country where vote choices are now fragmented, and may be further disrupted by the new divides over Brexit. Patterns of switching to and from Ukip could prove crucial. May’s embrace of Brexit has made the Conservatives more attractive to the 3.9 million voters who backed Ukip in 2015, while socially conservative, “left-behind” traditional Labour voters may be more attracted to Ukip as an outlet for protest votes against Corbyn. Angry and energised Remain voters may switch from both main parties to the Liberal Democrats in seats where the Lib Dems have little chance of victory, with unpredictable effects on the result.

These complicated dynamics may be magnified in open seats, as retiring MPs lose their personal vote and local loyalties give way to national trends. Swings could be unusually large and variable, depending on how these shifts combine locally. Apparently safe Labour seats could be jeopardised if Labour simultaneously leaks Remain votes to the Liberal Democrats and Leave votes to Ukip, while the Conservatives squeeze a large 2015 Ukip vote. Conservative seats could fall against the tide if Remain-voting Tories desert May’s Brexit agenda in large numbers.

Such speculations highlight the larger question hanging over the Brexit election: will this be a “politics as usual” election, with large but predictable swings of the electoral pendulum, or will the divisive debate over Brexit trigger more fundamental and unpredictable shifts in voting behaviour? While the former looks more likely, the latter can’t be ruled out – voters, like prime ministers, have not lost their capacity to spring surprises.