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Nick Clegg has described Labour as "increasingly irrelevant" and suggested voters face a straight choice between his Lib Dems and the Conservatives.He said it was "a nonsense" that Gordon Brown could end up "squatting in No 10" just because of the UK's voting system.
Meanwhile, Lord Mandelson has warned Labour voters that switching to the Lib Dems could hand victory to the Tories.
It comes as four opinion polls suggest the Conservatives remain in the lead, albeit still short of a majority.
Leader David Cameron is due to attack Labour on education later.
'Idiosyncrasies'
In an interview with the Sunday Times, Liberal Democrat leader Mr Clegg described the possibility that Labour could get a smaller share of the vote than the other parties, but still have more seats, as "a complete nonsense".
"You can't have Gordon Brown squatting in No 10 just because of the irrational idiosyncrasies of our electoral system," he said.
He said Mr Cameron would be making a "massive strategic error" if he opposed a reform of the voting system.
Mr Cameron is strongly opposed to proportional representation but a source told the BBC that if voters returned a hung parliament, he would be "constructive".
Meanwhile, Labour and the Tories have turned their fire on each other after a week in which they had both attacked Lib Dem policies following that party's jump in the polls.
The prime minister stepped up his assault on the Conservatives by hitting out at their pledge to raise the inheritance tax threshold to £1m, in an interview with the Independent on Sunday.
He said: "How can it be a priority to give to people who have already got so much?"
Despite Labour's position in the polls, Mr Brown insisted: "I am a fighter. When things are tough, you've got to get up in the morning and show some resilience and determination and fight through it."
Mr Cameron is expected to use a rally in Yorkshire to attack Labour for leaving children stuck in "sink schools" and spell out how his party would give parents, charities, churches and social entrepreneurs the chance to set up new schools.
His party has unveiled its latest poster campaign, featuring the leader - sleeves rolled up in front of supporters - along with slogans such as "Let's scrap ID cards" and "Let's fund new NHS cancer drugs". And shadow chancellor George Osborne signalled a change in strategy by telling activists in an e-mail that they can "expand the battleground" to Labour's heartlands.
New targets include Morley and Outwood, in West Yorkshire, where Schools Secretary Ed Balls is defending a notional 8,669 majority, Thurrock, Dagenham and Rainham, in Essex, and Middlesbrough South and East Cleveland.
"Labour's collapse has opened up new opportunities for us that we will be making the most of in the week ahead," his e-mail said.
Business Secretary Lord Mandelson insisted the election was "wide open" but acknowledged that some former Labour voters were considering "looking elsewhere".
He told the Sunday Mirror: "You might start flirting with Nick Clegg, but that way you will end up marrying David Cameron.
"If they vote Lib Dem, they are making it easier for the Tories to get in."
Meanwhile, more than 45 TV stars, including Harry Enfield, Jo Brand, Eddie Izzard and former Dr Who star David Tennant, signed a letter to Sunday's Observer criticising Tory plans for the BBC. They accuse the party of "attacking the BBC to serve the interests of its commercial rivals".
However, a party spokesman dismissed the claim as "complete nonsense", saying the Conservatives had "continually expressed our support for a strong and independent BBC".
Polling figures
Editor of the BBC's political research unit, David Cowling, said the latest polls for Sunday's newspapers suggested the Conservatives could narrowly nudge ahead of Labour as the largest single party in the Commons but still not secure an overall majority.
A ComRes poll of 1,006 adults on Friday and Saturday for The Independent on Sunday and Sunday Mirror showed the Tories on 34% - up three points on last Sunday's poll - the Lib Dems in second place with 29% (no change) and Labour third on 28% (up one).
For the Sunday Times, a YouGov poll of 1,412 voters conducted on the same days put the Conservatives up two points on last week at 35%, the Lib Dems down one on 28% and Labour down three on 27%.
Meanwhile Ipsos-Mori - for the News of the World - put the Tories up four points on 36%, Labour up two on 30% and the Lib Dems down nine points since Monday, on 23%. It surveyed 1,245 adults on Friday.
An ICM survey for the Sunday Telegraph put the Tories on 35% (up two points on last Sunday), Lib Dems on 31% (up one) and Labour on 26% (down two).This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.