The three leaders have all been out campaigning on Monday
David Cameron has said the Tories are to target Labour strongholds, in the wake of the Lib Dem poll surge.
He said Conservatives were "enlarging our battleground" but denied it meant they had given up on Lib Dem seats.
But Labour said the Lib Dem surge hurt the Tories more and stepped up attacks on their public services policies.
Lib Dem leader Nick Clegg said he would work with anyone backing his priorities but said voting reform was an "absolute pre-requisite for renewal".
In other election developments on Monday:
On Monday Mr Cameron set out Conservative policies telling voters concerned about the environment and protecting civil liberties: "There is now a modern Conservative Party that can get things done."
He denied being forced into a change of strategy after the Lib Dem surge following the prime ministerial TV debates - which polls suggest has pushed Labour into third place.
But he said it provided his party with "a great opportunity" to "extend our range of targets in the north of England and across the Midlands" - in seats where Labour has enjoyed healthy majorities.
"What we have seen, because of the meltdown in the Labour campaign, is an opportunity to work even harder in certain seats where frankly it's always felt that they are quite safe Labour seats, but they are not any more, they are up for grabs."
He denied giving up on Lib Dem seats and said the Conservatives were "enlarging our battleground".
Both Mr Cameron and Lib Dem leader Mr Clegg faced repeated questions about whether they could work together, if, as polls suggest, the general election on 6 May resulted in a hung parliament.
Mr Cameron played down suggestions he would negotiate over changing the first-past-the-post system to win Lib Dem support saying he wanted to "stand by the current system" because it allowed voters to "throw out" an unpopular government.
He said Mr Clegg wanted to "hold the country to ransom" over his demand to change the voting system.
Asked if proportional representation was a pre-requisite for Lib Dem support, Mr Clegg said "electoral reform is a first step" for any government to "start restoring public trust in the political system.
Asked if he could work with Mr Cameron, the Lib Dem leader said: "I will work with anybody, I don't care what label they have got attached to them, who will join with me in seeking those big changes in Britain. It's quite clear from what we have heard this morning that David Cameron wants to set his face against any serious change in our politics. I believe in a new politics, he believes in an old politics."
Mr Clegg also said he would not support Gordon Brown as PM, if Labour came third in terms of number of votes but still won the most seats under the first-past-the-post system.
However in an interview with Sky News he did not rule out working with other senior Labour figures like Alan Johnson or David Miliband.
In an interview to be broadcast on BBC 3 on Wednesday, Mr Brown said it was "strange" that politicians were "spending their time speculating about what happens after the election" rather than putting policies to people.
"Don't presume what the people are going to vote before they vote... because that would be arrogant," he said.
Labour's election co-ordinator Douglas Alexander said: "My sense is that Nick Clegg has somewhat overreached himself, maybe intoxicated by the publicity he has received in recent days, in getting into the speculation business."
He said Mr Clegg's "hostility" to Labour made it easier for them to tell voters in Labour/Conservative marginals: "If you vote Clegg, you'll end up with David Cameron."
He said some seats which Labour had expected to be "tough" were now "back in play, as a result of a significant rise in Liberal Democrat support" and dismissed Mr Cameron's comments as "spin".
Labour is launching its health manifesto, outlining patient guarantees and warning against what they call the Tories' plan for "do-it-yourself public services", which they see would see "a return to long waits" in the NHS.
And Schools Secretary Ed Balls said comments by two senior Conservative councillors about Tory education policy had "let the cat out of the bag" by querying whether it would drain funds from mainstream schools.
Mr Brown dismissed reports of Cabinet splits over Labour's election strategy as "innuendo and gossip".