Nick Clegg has claimed the election posed the biggest choice for voters in a generation as he promised to make a "full-throated appeal" for liberal values.
The Liberal Democrat leader said he was stepping up his campaign in order to counter the risk of the country taking a "wrong turn" on polling day.
The UK's Deputy Prime Minster said it would be a "disaster" if the party split under the pressure to support a Tory government in another hung parliament.
His comments in The Independent on Sunday came as another clutch of opinion polls showed the Conservatives and Labour vying almost neck-and-neck with neither likely to be able to form a majority government after the election on Thursday.
Video: 326 the magic number in UK general election https://t.co/bjrDDKp4vv
— RTÉ News (@rtenews) May 3, 2015There are reports of deep misgivings among many Lib Dems over the prospect of a second pact with the Tories.
Of particular concern to the Lib Dems is the fact that David Cameron is committed to a referendum on Britain's membership of the European Union, which they bitterly oppose.
Mr Clegg acknowledged the party had paid a high price for joining the Conservatives in government after the last general election in 2010 but said that did not mean there should be a "fatwa" against future coalitions.
Mr Cameron is to kick off a final push to election day with a direct appeal for Lib Dem and Ukip supporters to vote tactically to keep Ed Miliband and the SNP out of Downing St.
'He who dares wins' - Listen to a report by London Correspondent Fiona Mitchell
Labour leader Ed Miliband spoke about trust with just four days left until the UK goes to the polls.
He said: "Nick Clegg and David Cameron have helped erode trust in all political leaders by the way they broke promises on issues like tuition fees and immigration after the last election.
"If I am prime minister I will keep our stone in a place where we can see it every day as a reminder of our duty to keep Labour's promises."
He has a full day of campaigning planned today, taking him from the West Midlands to London.
Mr Clegg will be in London, before taking his message to Kent.
Green leader Natalie Bennett will also campaign in the capital, canvassing with party candidates.
In Scotland, All four main party leaders are gathered for a BBC debate in Edinburgh.
In the latest opinion polls, ComRes for The Independent on Sunday and Sunday Mirror puts Labour and the Conservatives level pegging on 33% with Ukip on 13%, the Lib Dems on 8% and the Greens on 7%.
Opinium for The Observer also has the two biggest parties almost neck-and-neck with the Tories on 35% and Labour on 34% with Ukip on 13%, the Lib Dems down on 8% and the Greens on 5%.
In the The Sunday Times, YouGov has the Tories on 34%, Labour on 33%, Ukip on 13%, the Liberal Democrats on 8% and the Greens on 5%.
Survation in The Mail On Sunday has Labour on 34%, three points ahead of the Conservatives on 31% with Ukip on 17%, the Lib Dems 8% and the Greens 4%.
