Polls continue to point towards a hung parliament or a small Tory majority, and there has been increasing talk of voting tactically - not for the voter's first choice but for another party to keep a more disliked rival out.
The BBC's head of polling, David Cowling, said tactical voting had not really determined the outcome of previous elections.
But he said this contest was so close that the decisions of voters in 20 to 30 constituencies could make all the difference.
The Labour-supporting Mirror newspaper has devoted its front page to a tactical voting guide aimed at "stopping" a Conservative win.
Cabinet minister Tessa Jowell told the BBC that was a "good thing" but added: "Ultimately, the point is, people make up their own minds."
Official Labour policy is to support its candidate in every seat they are fighting.
Mr Balls, who is facing a Conservative challenge in the Morley and Outwood constituency, stopped short of saying Labour voters should vote Lib Dem in Lib Dem/Tory marginals.
But he told the New Statesman: "I always want the Labour candidate to win, but I recognise there's an issue in places like North Norfolk, where my family live, where Norman Lamb (Lib Dem) is fighting the Tories, who are in second place. And I want to keep the Tories out," he said.
I think it's important for people to act intelligently in this election
In Tory-Labour marginals, he urged Lib Dem supporters to "bite their lip" and vote Labour.
Later he told journalists he was not going to criticise people who make their own judgements to keep the Conservatives out, including in Norfolk North.
Meanwhile Welsh Secretary Peter Hain told BBC Radio 4's Today programme: "I want every Labour candidate to win but many are not going to be in a position to win.
"I think it's important for people to act intelligently in this election."
He told the Independent voters "know what the real fight is in their own constituency" and would "draw their own conclusions".
But he denied this meant Labour voters should back the Lib Dems in some areas. He said it was in the "100 or so Labour-Conservative battlegrounds" where it would be decided which party won a majority.
In an interview with GMTV, Gordon Brown was asked if he would urge people to vote tactically, but said: "No, because I want people to vote Labour and I want a majority Labour government."
David Cameron dismissed pleas for tactical voting by Labour
Labour's election co-ordinator, Douglas Alexander, said that if the "anti-Conservative" vote was split between Labour and the Lib Dems, it could "let in" the Conservatives.
Former Lib Dem leader Lord Ashdown said the Labour calls were "patronising" and "desperate".
But Justice Secretary Jack Straw told the BBC the Lib Dems had used tactical voting more than any other party over the years, and denied it was a sign of desperation.
Campaigning in north London, Mr Cameron told reporters: "What seems to be happening this morning is senior Labour politicians are saying that if you want to keep Gordon Brown in Downing Street you vote Lib Dem.
"That backs up what we have always said - if you want on Friday a new government that rolls up its sleeves, starts to clear up the mess, then you need to vote Conservative on Thursday."
Shadow business secretary Ken Clarke said Mr Balls was "obviously getting very worried".
"I don't want to alarm him any further but if there's tactical voting in this election it's disillusioned Labour voters who can't stand Gordon and Ed voting for Liberal Democrats, rather than Conservative voters," he said.
The Lib Dems said ministers should not try to "second-guess" the public, saying Labour's tactics were symptomatic of the "old politics" that they were fighting against.
"I don't think any politician should tell you how to vote," Nick Clegg said.
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