Nick Clegg has warned against a coalition that would see Nigel Farage or Alex Salmond walk through the doors of Number 10 as he launched his party's manifesto.
The Liberal Democrat leader said that come 8 May either David Cameron or Ed Miliband would be prime minister but they would not win a majority government and would not hold the "balance of power".
He said it could be Mr Farage, it could be the SNP's Mr Salmond or it could be him and the Liberal Democrats who would provide "an insurance policy against a government lurching off to the extremes".
Mr Clegg said only his party could "add a heart to the Conservatives, and a brain to Labour" and only the Lib Dems could help guarantee the right path between the excessive cuts of the Tories and the excessive borrowing of Mr Miliband's party.
The Lib Dem leader said his party would bring "prosperity for all".
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He warned voters that a Miliband/Salmond coalition would lead to "reckless borrowing" and urged them to keep Mr Salmond out of Westminster by voting in the Lib Dem candidate in Gordon.
And he said: "Imagine for a moment… what will become of our wonderful country in the next five years if Farage gets in."
The Liberal Democrats are expected to win between 20 and 40 seats at the General Election and could again play the role of kingmaker as neither Labour nor the Tories are expected to win a majority.
Unveiling his party manifesto at a trendy art space in Battersea, Mr Clegg set out five deal-breakers for any future coalition cautioning against a lurch to the Left or the Right.
He attempted to persuade voters he could be trusted after the U-turn on tuition fees, which cost the party so much support in the early days of the coalition.
The five pledges on the front of the 160-page manifesto are:
:: Ringfence the education budget from age 2-19
:: Additional £8bn a year funding for NHS by 2020
:: Eliminate deficit by 2017-18
:: Raise the income tax personal allowance to £12,500
:: Green laws including decarbonisation target for electricity
Three of them on the NHS, the deficit and income tax - match promises made in the Conservative manifesto.
Mr Clegg is hoping to persuade voters that his party can be the "proven rock of stability, continuity and conscience".
The Lib Dem leader is keen that people should remember the things the Lib Dems delivered in power - and not the things they could not.
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This includes lifting thousands out of income tax by increasing the personal allowance, a policy the Conservatives have claimed credit for and put at the centre of their 2015 manifesto.
Despite largely being viewed as the scapegoats for unpopular decisions, Mr Clegg, whose manifesto launch was marred by a technical glitch that saw him fall off air during the question session, said that every day in coalition had been worth it because they had helped to make Britain better.
And, like Mr Cameron, he implored voters to be allowed to finish the job.
The Liberal Democrat launch comes on the same day as UKIP's - the other party looking to appeal to the Conservatives in the event of coalition building.