The Conservatives today accused Nick Clegg of trying to "hold the country to ransom" after the Liberal Democrat leader spelt out some of the conditions under which he would offer his party's support in a hung Parliament.
Mr Clegg made clear that electoral reform would be an "absolute pre-condition" for his party, and indicated that he would not be willing to prop up Gordon Brown in power if Labour came third in the popular vote but won the most seats.
But there was doubt over whether the Conservatives would contemplate any post-election pact which involved ditching first-past-the post for Westminster elections.
David Cameron said he wanted to keep the existing system but avoided explicitly ruling out voting reform.
But he made it more difficult for himself to strike an eventual deal with Lib Dems by saying that proportional representation would be "a big, big mistake for this country" and describing their policies as "away with the fairies".
And the Tories launched an election broadcast warning that a hung Parliament would lead to back-room deals, indecision, weak government and a paralysed economy.
To drive home the danger of a Labour/Lib Dem coalition, the film featured a yellow noose against a red background as the symbol of the "Hung Parliament Party".
Meanwhile, Gordon Brown said it was "arrogant" for politicians to start discussing what deals they might cut before voters have even cast their ballots.
And Labour's election co-ordinator Douglas Alexander said Mr Clegg had "over-reached himself" because he was "intoxicated" by the unaccustomed attention he has received in recent days.
With opinion polls indicating the General Election remains a three-horse race, the issue of which party might secure Liberal Democrat support in the first hung Parliament since 1974 is growing in prominence.
Mr Clegg today appeared to indicate he could support a Labour Party which had slumped to third in the share of the vote, but said it would be "inexplicable" to the electorate if Mr Brown remained premier in these circumstances.
Pressed on whether he could work with other Labour figures, like Alan Johnson or David Miliband, the Lib Dem leader said: "I will seek, with whomever else, to deliver those big changes that I want, in the way the economy is run, the way the tax system works, the way our education system works and, of course, cleaning up politics from top to toe."
Mr Cameron said PR was a "great con" because it takes the power to choose the government away from voters and puts it in the hands of politicians.
While avoiding an explicit commitment to retaining first-past-the-post, he said: "I think it is a decisive way of changing our government... I do not want the electoral system changed."
Mr Clegg was "interested in one thing and that is changing our electoral system so that we have a permanent hung Parliament, we have a permanent coalition, we never have strong and decisive government", said the Tory leader on a campaign visit to Lib Dem marginal Romsey.
"It's now becoming clear he wants to hold the whole country to ransom... just to benefit the Liberal Democrats."
Mr Clegg said it was clear Mr 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. It's a clear choice."
Mr Brown sought to turn the campaign focus on to Labour's policy differences with the other parties, with a speech to the Royal College of Nursing in Bournemouth in which he reiterated health guarantees and promised to protect nurses' pensions, increase numbers of specialists and avoid an NHS pay freeze.
As Conservatives revealed they were "expanding our battleground" into formerly safe Labour seats, there were signs of disgruntlement among the Labour rank-and-file about the conduct of their party's campaign.
Dari Taylor, whose Stockton South constituency is one of the Tories' new targets, warned that activists "feel excluded" by the focus on "flashy" leaders' debates and said Cabinet ministers should be spending more time talking to voters.
One-time rebel Fiona Mactaggart revived speculation about a post-election leadership change, describing Mr Johnson as a "contender" for Mr Brown's job.
Meanwhile, Labour was forced to suspend its parliamentary candidate in South East Cambridgeshire, John Cowan, over allegations that he posted "unacceptable" messages online.
But Mr Brown insisted that his focus on policy was paying dividends, telling Sky News: "I think we have a newcomer today on the stage and it is policy.
"People are starting to look beyond the surface and they are seeing things that they don't like about the Conservative policy. They are starting to see that they are a risk to the recovery.
"They are starting to see that the Liberals are a risk to security and a risk to child tax credits.
"I think they are starting to see that we have come through a very, very difficult recession, we have taken some of the most difficult and sometimes unpopular decisions to get this country through it and we now have a chance of moving forward."