Tactical voting: How much can the Lib Dems help Labour, and vice versa?

Several sharp minds have noticed an important sea change in opposition politics: Labour and Liberal Democrat voters are coming together. As Ben Ansell notes in a recent analysis of how voters divide up, “there is a huge strategic opportunity for a Lib-Lab pact. The parties’ bases differ in their emphasis... But they are both firmly in the [same political space]…with people switching between them like bored Tinder users.” Paula Surridge has noticed that a lot of voters who are flirting with Labour now have backed the Lib Dems in the past . And John Curtice has suggested that, as in the mid 1990s, the government is now so unpopular with some parts of the electorate that voters “will take whatever stick is available to beat it with.”

The current context should encourage tactical switching between Labour and the Lib Dems. The two parties are close together on most issues, supporters of each realise this and regard the other as an acceptable second choice. And many supporters in both camps regard removing the Conservatives as a priority.

This could matter a lot at the next contest, in part because none of this was true in 2019. Many Lib Dem leaning voters were hostile to Jeremy Corbyn, suspicious of Labour’s Brexit policy and opposed to many of the opposition’s big spending commitments. The legacy of Clegg and Coalition left many 2019 Labour voters wary of the Lib Dems, and doubtful of the party’s progressive credentials. Conservative Remainers who disliked Boris Johnson were put off tactical voting for the Lib Dems by fear the smaller party would let Jeremy Corbyn into office. As a result there are a lot of voters who didn’t see much merit in tactical co-ordination then, but may see and respond to tactical incentives now. There are often many third placed Lib Dem votes to squeeze in Conservative-Labour marginals, and lots of third placed Labour votes to squeeze in Conservative-Lib Dem marginals. This creates real potential for tactical voting to impact the election result.

Tactical gains 1: Seats Labour could pick up with Lib Dem help

How big could this impact be? Lets’ start with Conservative vs Labour marginals, where the Lib Dems are in third place (or lower). Many of these seats are tight enough that they will fall anyway if the tide goes out for the Tories, whatever happens with tactical voting. We want to focus on the places where tactical voting can tip the balance after accounting for the likely overall swing. To do that, I apply a uniform national swing of ten points to Labour (so Conservatives down ten, Labour up ten, everywhere). All other parties’ shares are left constant. I also focus only on England and Wales - the tactical voting situation in Scotland is very different, and is something I’ll return to in a future post. Finally, this is all done on the constituency boundaries that applied in 2019. New boundaries are coming in for the next election, and will introduce further complications. We will return to these another time.

A ten point swing is lower than the implied swing in the current polls, but taking a lower figure is consistent with the historical tendency for governing parties to recover some ground as election day approaches - a tendency which is stronger for Conservative governments:

The Midterm Limbo Dance

A ten point England and Wales only swing also delivers an evenly divided Commons, one where tactical voting would therefore have the biggest political impact by altering the balance of power. Applying such a uniform swing in England and Wales, and changing nothing else, gets us to 307 Labour seats, 250 Conservative seats and Lib Dems on 23 seats: a hung Parliament, with Labour around 15-25 seats short of a working majority.¹ How much could tactical voting change this picture?

First, I look at the most marginal Conservative-Labour contests in our new Commons and see what share of the third place Lib Dem vote would need to go Labour for each seat to fall. This gives us a sense of how many extra seats Labour could in theory gain through ‘loaned’ Lib Dem votes, and what level of tactical voting they would need to deliver each gain. Table 1 presents the results, ranked by the share of the 2019 Liberal Democrat vote Labour would need to secure to win the seat.

Table 1: Extra seats Labour could gain with tactical Liberal Democrat votes in a 10 point uniform swing scenario

Tactical voting can indeed make a difference in quite a lot of seats. I have divided the table into three categories. The first category, in dark green, is the lowest hanging fruit: seats Labour could gain by winning a quarter or less of the 2019 Liberal Democrat vote. Even a modest level of tactical voting enables Labour to defeat an extra five Conservative incumbents, including several prominent figures - Grant Shapps, Chris Philp and Conor Burns.

The medium green indicates seats which would fall if between a quarter and a half of local 2019 Lib Dem supporters tactically switched to Labour, along with a 10 point Con-Lab swing. Labour’s gains now rise to fifteen seats, and the opposition fell some further big beasts including Jacob Rees-Mogg and Liam Fox. Finally, the light green adds in seats Labour could gain by squeezing between half and three quarters of the Lib Dem vote - this adds in another 11 seats where the Conservative incumbent could be vulnerable to an intensive and unusually successful tactical voting squeeze.

All told, more than 20 additional Conservative seats would be vulnerable with large scale Lib Dem to Labour tactical voting on top of a ten point swing. These extra seats could be crucial in a tight contest - indeed in our current scenario they would take Labour from minority to majority status in the Commons.

Tactical gains 2: Seats the Lib Dems could pick up with Labour help

The Lib Dems also stand to gain by squeezing third placed Labour votes, but they can also benefit by diverting some of the national Conservative to Labour swing their way. Disgruntled Tory voters may switch to the Lib Dems rather than Labour when the former are best placed to defeat a local Conservative incumbent. Disaffected socially liberal, Remain leaning Tory voters who refused to back the Lib Dems in 2019 due to the risk of letting Jeremy Corbyn into Downing Street do not have the same anxiety about the prospect of a Keir Starmer led government.

To estimate Lib Dem tactical voting opportunities, I start with the same ten point national Conservative to Labour swing as before. But now, in seats where the Lib Dems came second, I make a second adjustment. The Conservatives still lose ten points, but in these seats the ten point gain is split evenly between Labour and the Lib Dems. I therefore assume about half of the Conservative to Labour swing voters vote tactically for the Lib Dems where it makes sense to do so locally. This step alone is enough to deliver five extra seats to the Lib Dems, all in London and its surrounding suburbs - Cities of London and Westminster, Finchley and Golders Green, Hitchin and Harpenden, Wokingham and Surrey South West - Jeremy Hunt’s seat, where the Lib Dem vote rose by a whopping 29 points in 2019.

Having handed local Lib Dems half of the national swing, we now repeat the same exercise as before - ranking seats in terms of what proportion of the third placed Labour vote would need to tactically switch to put the Lib Dems over the top. Table 2 shows the seats in play.

Table 1: Extra seats the Lib Dems could gain with tactical Labour votes in a 5 point Con to Lib Dem swing scenario

The Lib Dems can put an extra half a dozen seats in play in this scenario by winning over a quarter or less of the third placed Labour vote. Top of the list is Sutton and Cheam, where I grew up. Tactical voting helped defeat my local Conservtive MP in 1997, the first of many elections I have stayed up all night to watch. Even a modest tactical swing in 2024 could be enough for history to repeat 27 years later. Bigger squeezes could deliver further gains, with a total of thirteen seats available to the Lib Dems if they can persuade three quarters of local Labour voters to switch, including David Cameron’s former seat of Witney.

The Lib Dems start with 23 seats in our scenario, gaining a dozen seats on 2019 just from the national swing against the Tories. If half the anti-Tory swing is diverted to Lib Dem challengers, they pick up another five seats. Squeezing third placed Labour votes could deliver another ten or more on top of this, putting them close to 40 seats.

Putting our sums together, what do we get? Without tactical voting, we have a very hung House of Commons - with Labour on 307 and the Lib Dems on 23, the two parties combined would have 330 - a very narrow and unstable majority of 10. But if half of the voters with the strongest incentives to vote tactically loan their votes to the strongest local challenger, then these figures rise to Labour 322, Lib Dems 38, two parties combined 360. Labour are now very narrowly short of a majority, while Labour and Lib Dems combined have a solid majority of 70. If Labour and Lib Dem voters co-ordinate effectively at the local level, they could enable their parties to form a dominant alliance at Westminster.

When would tactical voting matter most?

So far we have taken one fixed national scenario - a ten point Labour to Conservative swing - and looked at what outcomes tactical voting could influence given that swing. But what happens when we vary the swing? When would tactical voting matter most? We once again need to break this down into two aspects. For tactical voting by third placed Lib Dems in Conservative-Labour contests, I applied national swings varying from 5 points to 15 points in England and Wales, took at look at the new winners in each seat, then took at look at how many additional seats Labour would win if they could bring over half of the local third placed Lib Dems. The results are in figure 1 below.

Figure 1: Labour seat totals at different levels of swing in England and Wales, without tactical voting (dark bars) and with 50% tactical switching by Lib Dems (light bars)

Labour can make substantial gains from tactical voting at every level of swing. The extra seats on offer vary from 13 to 20 seats through most of the range, but then really take off at the top end. In the event of a landslide level Labour swing of 14 or 15 points in England and Wales, large scale tactical switching from the Lib Dems could deliver 30 extra seats or more, turning a solid Commons majority into a dominant majority, and reducing the Conservatives to well below 200 seats.

Looking at how tactical voting impacts on Lib Dem seat totals requires some extra steps. I start by varying the Conservative to Labour swing, as before. Then, as in the first exercise, I divert half of that swing to the Liberal Democrats in the seats where they are local challengers to the Conservatives. Finally, I also give the Lib Dems half of the third placed Labour vote in those seats. This way we can see how both aspects of tactical switching - Conservative to Lib Dem and Labour to Lib Dem - impact on Lib Dem prospects at each level of swing.

Figure 2: Lib Dem seat totals at different levels of swing in England and Wales, without tactical voting (dark bars) with 50% tactical switching by Conservative switchers in Con-LD seats (medium bars) and with 50% tactical switching by both Con switchers and 3rd place Lab voters (lightest bars)

The gains the Lib Dems can make from tactical switching increase as the overall swing increases. This is in part by design - larger swings mean my model diverts more disaffected Tories into the Lib Dem column in Con-LD contests. But this also reflects a real feature of the current polling landscape - right now, the big change since 2019 is a large swing from Conservative to Labour, with the Lib Dems treading water in national polls. Lib Dem performance on election day will depend heavily on getting swing voters to behave differently in seats where they are the local challenger. Convincing unhappy former Tories and third place Labour supporters that the Lib Dems are “winning here” brings double digit seat gains once the national Con-Lab swing hits 8 points. If high levels of tactical voting combine with a massive anti-Conservative national swing, the Lib Dems could gain 20 or more MPs, an advance not see since 1997, which was also the last election with both of these features.

All of this modelling involves a bunch of simplifying assumptions - a uniform swing applied everywhere with no personal votes or local variations, and uniform rates of tactical voting regardless of local circumstances and campaigning. These are of course unrealistic - the real results will be more complex and variable than this. But this simple toy model is useful for getting a handle on how much tactical co-ordination between Labour and Lib Dem voters might matter in different election scenarios. It matters a lot, potentially delivering dozens of extra Conservative gains. These could be more than sufficient to change the political outcome in a closely divided Commons.

Nor is Lab-Lib Dem tactical co-ordination the only form of tactical shift with the potential to impact outcomes. Scottish Conservative leader recently suggested Scottish Conservative voters should “do what is best for the country” and back “the strongest candidate to beat the SNP”. Such Unionist tactical voting could prove very important in many Scottish seats with modest SNP majorities and fragmented opposition votes. ReformUK leader Richard Tice has also vowed to reverse his predecessor Nigel Farage’s 2019 decision not to stand candidates in Conservative held seats. This means new ReformUK candidates could appear to split the Conservative vote in such seats, even as opposition votes are coalescing. I will consider the potential impact of unionist tactical voting, and of local right splits, in future posts.

Small edit made to this post after publication to correct the spelling of Grant Shapps and Chris Philp’s names

1. In theory, in a 650 seat House of Commons, a party has a majority once it reaches 325 seats. In practice things are a bit more complicated - Sinn Fein MPs do not take their seats, while the Speaker of the House and the three deputy speakers do not participate either. Very narrow majorities are also not much different from hung parliaments in practice, as they are hostage to tiny rebellions and to events such as death and scandal which can and very often do reduce a governing party’s effective number of votes. The point where a ‘working majority’ which enables governments to get legislation passed is therefore often a bit fuzzy, hence the use of a range