Why the 2017 election was much worse for Theresa May, and much better for Jeremy Corbyn, than it looked

To understand the 2017 result, you need to understand the 2015 one. 

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There is a lot stuff talking about the election as if Theresa May did quite well – and a lot talking about what Jeremy Corbyn did right, focusing on his vote share rather than his more important gain, vastly increasing the number of winnable seats.

The 2015 election result was so bad for Labour, as I wrote immediately after, before Corbyn was even on the ballot, that they had lost two elections in one night.

Seats like North Swindon, Labour-held until 2010, had Conservative majorities in excess of 10,000. They needed a swing of 1997-proportions to get a majority of one, and to gain close to 100 seats. Their vote was badly distributed, going up in seats they held and retreating in seats they needed to win to form a government.

In 2017, the picture is very different. Labour needs a swing of just 3.5 per cent to win a majority of one, well within the reach of the possible. A swing of just 1.4 per cent from Conservative to Labour would gain the party 30 seats and put them in office in a minority administration – not one in which they could carry through all of Jeremy Corbyn’s manifesto, but one in which they would be able to easily govern from a centre-left perspective with the help of the nationalist parties and the Liberal Democrats.  

In contrast, to get into minority administration territory after 2015, Labour needed close to a six-point swing. Just 15 seats would have fallen into Labour hands on a 1.5 per cent swing, well below the 33 seats that Labour gained on 8 June.

Vote share is not all that relevant under first past the post. What matters about Corbyn’s performance as opposed to Ed Miliband’s is that his vote is efficient under first past the post and he has created an electoral map where Labour can honestly talk about winning the next election. (Regardless of who had won the Labour leadership in 2015, their best case scenario was not victory but creating the conditions for victory at the election after that, as I wrote at the time.)

Nor is vote share that useful a metric to talk up May’s dire performance. Ultimately, there were four reasons why Theresa May and Jeremy Corbyn got such a high vote share: 1) The collapse of Ukip following the referendum, 2) The failure of the Liberal Democrats to revive in a big way, 3) Voters who had never voted until the referendum continuing the habit in the election, and 4) First-time voters motivated by Corbyn.

Neither May nor Corbyn can take credit for the first three, and only Corbyn can take credit for the fourth.  The others were the result of decisions taken by, variously: David Cameron, Nick Clegg, Tim Farron, Nigel Farage and Paul Nuttall.

Most of the increase in vote share was about what happened to the minor parties. Their collapse and failure increased the size of the marketplace for the big two.

And on this metric you get a better idea of Corbyn’s success and May’s failure. Corbyn took 48.5 per cent of the Labour-Conservative combined score, up from 45 per cent under Ed Miliband. Theresa May in contrast took 51 per cent of the two-party combined score, down from 55 per cent under David Cameron.

I will write in more detail about the 2015 result, and what the 2017 result has done for the prospects of the four major parties. But the most important thing to understand is this: while there are legitimate arguments that 8 June 2017 might be “peak Corbyn”, the election result was a very good one for Labour and a very bad one for the Conservative Party. That has nothing to do with their expectations before the exit poll – and everything to do with the actual pattern of votes and seats across the country. 

Stephen Bush is special correspondent at the New Statesman and the PSA's Journalist of the Year. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics.