Tactical Voting Parameters

Revised 17-Dec-2020

The user-defined prediction also gives a (simple) model of tactical voting, which lets you control the size and direction of any effect.

The idea of the model is that some voters whose first-choice party is likely to come in third place would rather vote for their second-choice party which is more likely to win.

See the Tactical Voting Model for more details about the model itself and the historical experience.


In seats which the Conservative and Labour parties are quite close, and the Liberal Democrats are likely to be in third place, some Liberal Democrat supporters may switch their vote to another party. The fraction of Lib Dem voters which vote tactically in this way is assumed fixed at, say, 25% of all Lib Dem supporters in the seat.

We call this a 25% "tactical fraction" of Lib Dem supporters.

Input parameters

In the user-defined prediction entry page, there is an area for entering the tactical swing levels:

% % % % %

In this example, the user specifies a zero tactical fraction of Conservative voters, a 25% tactical fraction of Labour voters and a 35% tactical fraction for Lib Dem supporters.

Typical parameter sizes

The value of tactical swing parameters must be between 0% (no tactical voting) and 100% (maximum tactical voting). In practice, tactical fractions usually lie between 10% and 50%.

At the 2019 general election, there were significant amounts of both pro- and anti- Conservative tactical voting. In some seats, tactical voting was as high as 50pc, but in other seats it could be around 20pc.

Tactical preferences

Voters who vote tactically do not all pick the same destination for their tactical vote. It depends on which party they basically support, and also on the characteristics of the seat. For example, non-Green voters are unlikely to vote Green tactically unless the Green party is competitive in that particular seat.

The general trends of tactical preferences at the 2019 election were estimated as follows:

Additionally in Scotland, there was some pro-Union tactical voting against the SNP, but this is not captured by this model.

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