How Many Seats Could Reform Win, Really?
And why that is such a difficult question to answer
Hemmingway said bankruptcy happens gradually, then suddenly. So does winning seats in Parliament. Despite getting 14% of the vote last summer, Reform only secured a handful of MPs (and they are indeed a handful). But the party has surged since and at the time of writing the Politico poll of polls puts its support at 26% – enough to win a lot of seats all of a sudden.
This curve shows how many seats Reform could expect to win for a given level of swing (based on a crude assumption of the party gaining ground equally across every constituency in the country, at the expense of both Labour and the Conservatives). As its share of the vote increases above the 14.3% it won in July, its projected seat count creeps up slowly at first. Then it goes gangbusters. The slope of this curve is greatest around a 12 percentage point swing, equivalent to a vote share of 26%. In other words, a boost in the polls counts for more in terms of seats now than at any other level of support. Small shifts around this vote share could make the difference between a fine day for Reform and the collapse of the British party system.
So why does the relationship between votes and seats have such a dramatic functional form? The answer is a combination of the capricious British electoral system and the diffuse distribution of Reform voters.
The mechanics of first-past-the-post
Britain’s winner-take-all electoral system is cruel to small parties and kind to big ones. Or, as Nigel Farage recently told Sky News Australia:
“First-past-the-post can be your enemy, but there comes an inversion point at which it becomes your friend […] We are literally at that inflection point.”
He’s right. Reform was punished severely by the system last time, but now it is on the cusp of becoming one of its beneficiaries.
This chart shows the proportion of votes and seats won by parties in every general election since 1945. It’s easy to see that larger parties tend to win more than their fair share of seats while smaller parties win less. The last election – the most disproportionate ever – offers a striking illustration. Reform won more than 14% of votes but less than 1% of seats; meanwhile, Labour took almost two thirds of seats on one third of the vote. That’s around 23,500 Labour voters for each Labour MP compared to more than 800,000 Reform voters for each Reform MP.
The threshold where first-past-the-post begins to work in a party’s favour is around 26%. Below 26% vote share, the only parties to win a proportionate number of seats are those which stand just in Scotland, Wales, or Northern Ireland, whereas parties above 26% tend to win an outsize number of seats. On its current level of support Reform straddles that divide, which is part of the reason its projected seat count is so volatile. The rest of the explanation has to do with how support for Reform is distributed around the country.
Reform voters are scattered
First-past-the-post does not penalize all smaller parties equally. The Liberal Democrats received slightly fewer votes than Reform at the last election, but won 72 seats compared to Reform’s five. The discrepancy is down to geography.
Votes for Reform were spread thinly around the country. In almost every constituency, the Reform candidate did decently but not well enough to take the seat, or even to give the winner a scare. The exceptions were mostly high-profile figures like Farage in Clacton, Lee Anderson in Ashfield, and Richard Tice in Boston and Skegness.
The Lib Dems, in contrast, managed their vote extremely efficiently, which allowed them to win most of the constituencies where they were even remotely competitive. By ruthlessly targeting vulnerable Conservative seats and towns with posh bakeries, they turned a modest vote share into a record windfall of seats.1
The downside of the concentration of the Lib Dems’ vote is that there are very few winnable seats left for them to gain at the next election. Even if their support were to surge, they would probably eke out thirty extra seats at most while racking up massive majorities in constituencies they already hold. In contrast, an election held tomorrow would deliver a barrage of new Reform MPs, precisely because the party’s support is so diffuse. For small parties, the higher the concentration of their vote the better. But for large parties, a more uniform distribution is preferable because it allows them to win narrow pluralities in many constituencies rather than landslides in just a few. As Reform grows from a minor party into a major one, the relative uniformity of its support turns from a liability into an asset.
Can anything stop Reform?
The very same forces which hindered Reform when it was polling in the mid-teens – first-past-the-post and the uniformity of its support – are beginning to give it an edge. If it really does win a quarter of votes in the next election, it will probably win close to a quarter of seats too. If it’s support continues to rise, it could win even more than its fair share of seats. But the next election is (almost certainly) a long way off yet, and there are reasons to be sceptical about Reform’s prospects.
For one thing, Reform’s position in the polls might not be quite as strong as it looks. Most pollsters (including Deltapoll) slightly overestimated Reform at the last election. Paula Surridge has suggested that the polls might be overstating its support again because undecideds (who are excluded from headline figures) are very unlikely to have voted Reform in 2024.2 That might only make a few percentage points of difference to vote share but small fluctuations at this level matter a lot for seats.
Equally, Reform’s vote share might not translate into seats as the simplest models project. One way the assumptions of these models could be violated is if there is widespread tactical voting against Reform. There is some evidence that strategic voters could give Labour an advantage over Reform in marginal seats, although these dynamics won’t become clear until the election looms into view.
Even setting aside complications like tactical voting, predicting how many seats Reform will win is unusually tricky. Based on their trajectory in the polls, it really is possible that Nigel Farage could become Prime Minister. But it’s also plausible that Reform ends up as the fourth biggest party in Parliament. The strange thing is that these divergent outcomes are actually compatible with very similar vote shares for Reform. When margins of victory in elections are narrow, margins of error in forecasts are wide.
For those who prefer mathematical explanations to graphical ones, the Lib Dems’ Party Nationalisation Score (1 - Gini) is 0.48 compared to 0.71 for Reform. For context, a party with perfectly uniform support gets a score of 1, whereas a party that wins all its votes in a single constituency scores 0. Thanks to Chris Prosser for suggesting this metric.
This is for two reasons. First, Reform supporters are more likely to be male and men are less likely to say they don’t know who they will vote for. And second, Reform supporters in general - male and female - are more enthusiastic than supporters of other parties.
The R code used to produce the seat projections for this piece, along with much of the code used in this blog, is available on GitHub.