This post was written in collaboration with Martin Baxter of Electoral Calculus.

The Thames Valley constituency I live in voted for Remain in the EU Referendum, which is a little bit awkward because our MP is Theresa May. Surrounded by miles of natural beauty, Maidenhead is a concrete wart on the landscape – fashionable in its day, but now something of an embarrassment.

Like a trapped party-goer leaning politely away from a bore with bad breath, the Prime Minister lives about as far away from our town as it’s possible to get without actually leaving the constituency. With a majority of over 29,000 though, she’s unlikely to leave any time soon, even with the threat of another party contesting her seat: Howling Laud Hope of the Monster Raving Loony Party.

Still, it raises an interesting question that’s been asked by pundits and hopeful activists for months now: could Conservative MPs with less daunting majorities be vulnerable to a ‘Brexit effect’ in this election? Could a surge for the pro-European Liberal Democrats save progressives from a miserable night?

To find out, I teamed up with Martin Baxter, from Electoral Calculus. We wanted to take the best, most recent polling available on the voting intentions of remain and leave supporters, split by who they voted for in 2015, create a model of which voters are moving where, and apply that seat-by-seat to figure out what the major shifts are and what their impact could be in an election.

Our results depend on the accuracy of ICM’s polling of course, and they’re a description of the state of play now rather than a prediction of the final election result. Those caveats aside, it’s fair to say that they aren’t what we expected.

For raw data, we combined the three ICM polls commissioned by The Guardian during April (two after Theresa May’s snap election announcement, one just before), questioning a total of 3593 people from April 14th to April 24th. You can find the raw data on ICM’s website, here.

There are other polls out there, but ICM were the only pollster to break down their results by which party people voted for in 2015 and which way they voted in the EU referendum. This allowed us to see that, for example, 2% of Conservative Leave voters polled intended to vote for UKIP in June. You can see the full results in the transition matrix below, with previous vote split by column and voting intention by row.

Transition table showing where voters have shifted since the 2015 election and Brexit referendum

To understand the results, let’s imagine 100 perfectly representative Remain voters. 31 of them voted Conservative in the last election, 37 for Labour, 13 for the Lib Dems and 2, oddly, for UKIP. One of the Tories has switched to the Lib Dems, but the Conservatives have gained one each from Labour and UKIP – in other words, the Tories are gaining Remain voters, not losing them as some might assume. The Lib Dems are gaining too, but mostly from Labour.

Voting trajectory of 100 representative Remain voters.

Now let’s take 100 Leave voters. Here the picture is even starker, with the Tories sucking in votes like a right wing Dyson on steroids. A third of UKIP’s 2015 vote looks set to switch, followed by three of Labour’s 25 voters, and one from the Lib Dems.

Voting trajectory of 100 representative Leave voters.

That the Tories are gaining among both Leave and Remain voters goes some way to explaining their historic poll leads in recent weeks, but what could this actually translate to in terms of seats?



To find out, Other Martin used the transition matrix above to create a Markov chain style simulation. This approach takes a collection of possible states (i.e. being a Tory voter, being a Labour voter, and so on), and uses the probability that voters will hop between any two given states to simulate outcomes for a large population of voters. (For a superb visual explanation, check out this link.)

To make the model more realistic, we accounted for ‘stickiness’ of each parties core vote, dividing voters into the stronger “red ‘til I die” supporters and the weaker types more likely to swing. Put together, this provides a pretty good approximation of what would happen in an election if the polls were reasonably accurate.

Back in 1997, when New Labour won the general election in a historic landslide, Professor Anthony King described the exit poll as being like “an asteroid hitting the planet and destroying practically all life on Earth.” That election saw Labour win 418 seats to the Conservatives’ 165, while the Liberal Democrats gained 46 seats.

Our model sees the Tories on 422 seats, with Labour reduced to just 150, and the Lib Dems declining from 9 to 6. The Conservative majority would be north of 190. Labour would be wiped out beyond what most people are currently predicting. Leadership candidates like Clive Lewis would no longer be leadership candidates, because they would no longer be MPs.

The Lib Dems could lose a third of their MPs even after gains in places like Cambridge, with seats like Carshalton & Wallington, Richmond Park and Southport especially vulnerable. The danger in these seats is pretty clear. In Carshalton, Tom Brake won a majority of 1,510 in 2015. If a fraction of the town’s 7,000 UKIP voters return to the Tories, that majority will be wiped out. Southport is almost identically poised. Unless a major influx of Remain voters arrives from somewhere – and there’s no indication in any of this data that it will – then these seats will be lost. The Lib Dems don’t face the same problem in Richmond Park, which only turned back to the Lib Dems in December; but with a majority of less than 2,000 and a recent history of flipping, you wouldn’t bet the mortgage on a hold.

If all this sounds extreme, it’s in line with recent polling. In Wales, the Conservatives look set to beat Labour for the first time since the First World War, with the Tories rising 13 percentage points to 40%, and Labour falling 7 percentage points to 30%. The picture for Labour could in fact be worse than our model suggests – data from both ICM and the British Election Study data indicate that Labour is actually under-performing its polls in Labour seats. The Mirror quote a Labour source suggesting that private polling puts the party as low as 20% nationally. 150 MP may yet prove wildly optimistic.

But the bigger surprise is the lack of any real sign of a Lib Dem recovery. For months now, there’s been a growing consensus that the party are on the verge of a comeback. Try as we might, we could find little evidence of this in recent polls. It’s still early in the campaign of course. Polls could change, and tactical voting combined with effective local campaigning could make a difference in some seats.



It’s clear though that anyone taking a Remain surge for granted should probably dial down their expectations. So far this election isn’t looking good for anyone but the Tories.