P ERHAPS THE only view shared by Britain’s big parties is that backing the Liberal Democrats is a dire risk. “A vote for the Lib Dems gets you Brexit,” Labour warns. “A vote for the Lib Dems risks putting Corbyn in Downing Street,” claim the Tories.

Both sides cannot be right. However, survey data of 100,000 Britons from YouGov, a pollster, imply that both parties are wrong. Because the Lib Dems have pulled votes equally from their two rivals, further growth in their support would probably cost both Labour and the Tories seats.

With Labour neutral on Brexit, the Lib Dems are the main national pro-Remain party. Voters have noticed. YouGov’s data show that the few Leavers who backed the Lib Dems in 2017 largely plan to defect. But the party should pick up a fifth of the Remainers who voted Conservative last time, and 13% of Remain-supporting Labourites.

This has doubled the Lib Dems’ vote share, from 7% in 2017 to 14% in YouGov’s poll. But it may not yield many new seats, because Lib Dem voters are spread out geographically. YouGov matched personal data from respondents with the demography of each constituency to estimate voting results in every seat. The Lib Dems come first in just 13.

Jo Swinson’s party has fallen back in recent polls. However, late surges are common in British elections, particularly when tactical voting is widespread. How might the race change if the Lib Dems approach the 23% vote share they won in 2010?

To find an answer, we scaled up their popularity in every constituency to reach a scenario in which their national vote share was 23%. First, we grouped Britons based on their Brexit vote and whom they supported at the last general election—for example, Leavers who voted Lib Dem in 2017. According to YouGov, just 30% of these people plan to stick with the Lib Dems. To get to a national share of 23%, the party would need its support in this category to double. Next, we estimated how many voters in each group (such as Labour Leavers) live in each constituency, to determine the seat-by-seat impact of a Lib Dem surge.

In terms of winning seats in England for themselves, the Lib Dems pose a serious threat only to the Tories. There are 13 seats in which those two parties are the front-runners and are separated by a single-digit margin. Between the Lib Dems and Labour, the only close fight is in Sheffield Hallam.

However, the Lib Dems could still hurt Labour, by taking votes from the left-wing party and letting the Tories sneak through. This is especially likely in Tory-Labour marginals in the north and Midlands.

Which of these two effects is larger depends on tactical voting. We explored two endings for our hypothetical scenario: one in which Lib Dems surge uniformly, and one in which they disproportionately rally in seats where their former supporters have reluctantly flipped to Labour, hoping to prevent a hard Conservative Brexit.

If the swing is uniform, the Tories will lose out most, with perhaps 25 seats going from blue to yellow. If tactical Labour voters flock back to the Lib Dems, it will be Jeremy Corbyn who suffers more. But in both cases, late gains for Britain’s third party would leave the main two worse off. ■

Sources: Electoral Commission; British Election Study; YouGov; Chris Hanretty, Political Studies Review, 2019; The Economist