David Cameron has called for Lib Dem and Ukip supporters to vote tactically to prevent a Labour-SNP government coming to power.
Writing in the Sunday Times (£), the Prime Minister pleads with Lib Dem and Ukip voters to "use your vote carefully" without explicitly using the words "vote tactically".
"If you are considering voting Ukip or Lib Dem, I urge you to think of the chaos of a weak Ed Miliband, propped up by Nicola Sturgeon demanding ever more borrowing and more taxes," he writes. "Only a Conservative vote in your local constituency will keep Ed Miliband and the SNP out and secure Britain’s future."
History suggests this strategy will backfire. As Dr Stephen Fisher explains in Sex, Lies and the Ballot Box, tactical voting has previously hurt the Conservative party the most.
While the Conservatives often gain the largest number of tactical votes as they are rarely the third-biggest party in constituencies, they lose out in terms of seats under the first past the post system because of where other tactical votes go.
Lib Dem supporters favour voting tactically for Labour over the Conservatives, so Labour candidates get help in Con-Lab marginals. In Lib-Con marginals, meanwhile, Labour voters tend to support the Lib Dems. Dr Fisher and his colleague David Myatt estimated that in 1997 the Conservatives lost 35 seats to Labour and 11 to the Lib Dems due to tactical voting.
There aren't enough other kinds of seats to redress the balance for the Conservatives, and so overall, because of tactical voting, Labour and the Lib Dems both win seats from the Conservatives even though the Conservatives pick up more tactical votes overall.
Of course, this was before the coalition and the rise of smaller parties. But Dr Fisher predicts one effect of the greater uncertainty about the result of this election might actually be for there to be more mistakes in tactical voting.