What is tactical voting?
Tactical voting involves constituents who agree with the policies of one party deliberately choosing not to vote for their candidate.
Tactical voting has had a significant effect on past elections
Usually, this is because the candidate in question has little prospect of winning and so the voters prefer to give their second-choice options a better chance of winning the seat. In some cases, a decision to vote tactically may be taken because the voter particularly dislikes another party and wishes to support whichever candidate has the best chance of keeping that party's politician out.
When is it likely to happen?
Tactical voting usually happens in "marginal seats" - constituencies which are closely contested by two parties, with the rest trailing by some distance. It is less likely in seats considered "safe" for a particular party or where there is a true three or even four-way battle to get elected.
How many people vote tactically?
Dr Stephen Fisher, an expert in political sociology at Oxford University,
estimates up to 9% of voters mark their ballot papers tactically,
influencing the results of up to 45 seats. Another politics academic, the University of Strathclyde's Prof John Curtice, says tactical votes were worth as much as 3% of the vote - or 20 seats - to Labour when they swept to power in 1997.
What do Labour say?
Prime Minister Gordon Brown says he only wants people to put their cross in Labour's box although members of the party have been more explicit in saying they want Lib Dems to vote Labour in seats where there's a straight battle between Labour and the Conservatives. Cabinet ministers
Ed Balls and Peter Hain also hinted they might prefer potential Labour voters to switch to the Lib Dems
in seats where it might help to "keep the Tories out".
What impact will tactical voting have on this election?
Mr Hain, the Welsh Secretary, said his call for "intelligent voting" also applied to SNP and Plaid Cymru supporters - but nationalist politicians say a strong Westminster presence for their parties will have more bargaining power for their constituents.
What do the Lib Dems say?
Lib Dem leader Nick Clegg said Lib Dem voters should "vote for the change they want, the future they want, nothing else". Foreign affairs spokesman Ed Davey said Labour attempts to talk up tactical voting "smack of desperation", adding: "It's almost as if they have given up. The Liberal Democrats will be campaigning for every single vote. I think we are going to pull off some shock results."
What do the Conservatives say?
David Cameron said Labour ministers were saying that "if you want to keep Gordon Brown in Downing Street you vote Lib Dem". He said the only way to make sure there's a new government on Friday was to vote Conservative. Voting for any other party could lead to a hung parliament - perhaps with Gordon Brown still as prime minister. His party also said they expected people to vote tactically in some places to keep out Labour.
What about other parties?
Traditionally no party actually tells its supporters to vote for someone else. Doing so would undermine the efforts of their local campaigners and - in most cases - would break party rules. An exception this time are
UKIP, who openly called for the electorate in some areas to back eurosceptic candidates from other parties.