Will voters avoid choosing the weakest link in their constituencies in a bid to block their least favourite candidate?
By Andrew McFarlane
Transport Secretary Lord Adonis has urged Liberal Democrat supporters to turn to Labour in constituencies where their own party is unlikely to win.
Leading Lib Dems have rubbished the idea - but could such tactical voting play a big role in this year's election?
The prospect of voting for a candidate who is not your preferred choice may rankle with those who hold their democratic principles dear - a bit like voting off your strongest rival on The Weakest Link.
But, according to Prof John Curtice, tactical voting has been going on for years - and has played a key role in recent elections.
"In 1997 in particular it occurred on a very wide scale, typically being worth as much as 3% of the vote to the Labour party in seats that it was trying to win from the Conservatives," he said.
Based at the University of Strathclyde, he has been studying electoral behaviour for more than 30 years.
He said tactical voting benefited Labour to the tune of about 20 seats when they came to power, while the Lib Dems gained a dozen thanks to voters whose main aim was to oust the Conservative government.
That people vote against their convictions is a quirk of the UK's first-past-the-post system, which elects the candidate with the most votes regardless of his or her share of the polling.
Often, a candidate for a less-popular party "splits the vote" of one of the main contenders to which their political stance is most similar, giving an advantage to an opponent.
In an attempt to avoid just that, the Green party's candidate for Weston-Super-Mare, Dr Richard Lawson, has decided against standing in this general election and advised his supporters to back the Liberal Democrats.
"[First Past The Post] forces people to vote tactically, and in my case, to stand tactically," he wrote on his blog.
Dr Stephen Fisher, an expert in political sociology at Oxford University, believes the power of tactical voting could be even stronger.
Using theory to analyse results of post-election surveys, he estimates up to 9% of voters mark their polling cards tactically, influencing the results of about 45 seats in the process.
"It happens primarily when the third-placed party are quite far behind the top two, which tends to be the case in marginal seats.
"The people most likely to vote tactically are the ones who support a party coming third and have an affinity with one of the top two and really hate the other."
The last general election saw the emergence of a number of websites offering voters the chance to formalise this tactic by "swapping" votes.
The theory was that, for example, Lib Dem and Labour supporters could agree to vote for each other's parties in seats where their preferred choice stood little chance.
It is unclear how many stuck to that commitment or the impact those sites had.
Dr Fisher said that just because this election is promising to be the closest in years, it will not necessarily mean tactical voting is more common.
He says David Cameron's efforts to improve the Conservatives' image may have an impact.
Tactical voting has helped Labour in recent elections
"People who support the Lib Dems and previously switched to Labour might hate the Tories less - and might even prefer the Tories to Labour.
"That means Andrew Adonis's article may in some ways be going against the grain."
Prof Curtice agrees. He said it was less certain that Labour could rely on tactical voting than previously, with surveys showing 40% of Lib Dem supporters prefer Labour as a second choice but 30% the Conservatives - higher than previously.
"There are an awful lot of Labour MPs trying to defend marginal seats who... held their seats on account of people who might otherwise be expected to vote Liberal Democrat being willing to vote Labour in order to try to keep the Conservatives out," he said.
However, if that willingness falls away, it might not necessarily be of great advantage to the Lib Dems.
Dr Fisher points out: "It might not happen in place where it's useful for the Lib Dems because it would be in places where they are starting third."
Rather than helping Labour to hold seats, it could be the Lib Dems who have most interest in picking up tactical votes in areas such as south-western England, he says.
Against a backdrop of a resurgent Conservative party performing well in the polls, while the Lib Dem position has slipped, they might have to rely on support from Labour voters to keep their candidates in Parliament.
Prime Minister Gordon Brown's position was fairly unequivocal.
"If people don't want a Conservative government, then they must make sure they don't allow the Conservatives in," he said.
It is a far cry from the Labour's stance in 1997, when several activists were expelled from the party for doing just that by voting Lib Dem.