As working out the effect of potential tactical voting is causing some interest today, here's another example to understand the phenomenon.
In 2017, Survation polled the constituency of Bath, on behalf of the local Labour party.
Asked simply: If the General Election was tomorrow, which party would you vote for in your Bath constituency? Headline results were:
CON 32%
LAB 17%
LD 46%
AP 5%
However when we asked:
Now please imagine that at the next General Election the Conservatives, Labour and the Liberal Democrats all have an equal chance of winning in your Bath constituency. Which party would you vote for in that situation? We found: