David Cameron claimed that a Labour government would add £1,000 to the average mortgage as he said a Labour / SNP alliance would be the "worst" outcome of the General Election.
The Prime Minister said that a deal between Labour and the SNP would bring together those who want to break-up Britain with those who want to "bankrupt" it.
But he was accused of cynical politics after basing his mortgage claim on the effect of a one per cent increase in interest rates.
Critics pointed out that rates are expected to rise if Mr Cameron's Conservatives win May's General Election.
Interest rates have been at record lows in recent years, but Mark Carney, the Governor of the Bank of England, has suggested that they could increase next year.
That would be good news for savers, who have seen their nest eggs dwindle since the 2008 financial crisis.
In an attack on both Labour and the SNP, Mr Cameron told an invited audience at an event in Colchester, Essex: "If nobody thinks there is anything worse than a Labour government led by Ed Miliband, I have got news for you there is.
"There is a Labour government led by Ed Miliband and propped up by Alex Salmond and the Scottish National Party."
He added: "Don't laugh - because it could happen.
"We could have an alliance between the people who want to bankrupt Britain and the people who want to break-up Britain."
He also suggested that a Tory majority government would "safeguard the future of our country".
An SNP spokesman said: "A Tory Prime Minister sneering at the people of Scotland is something we are well used to - the most popular election outcome in Scotland is a minority Labour government with the SNP holding the balance of power.
"The people of Scotland certainly wouldn't be laughing at a Tory or Labour majority government - which would just mean more austerity and more Westminster cuts."
Labour leader Ed Miliband said Mr Cameron's housing plans plan had so far "failed Britain and failed families".
Mr Cameron's comments came as he pledged to build 200,000 cheap starter homes for the under-40s in England and Wales.
Aides to the Tory leader said that the policy was "one for the Scottish Government to think about".
Mr Cameron's appeal to the twenty and thirty-somethings follows accusations of "inter-generational theft" last month after ministers extended their controversial pensioner bonds policy.
Last week the Prime Minister also pledged to preserve benefits for older voters, including the Winter Fuel Allowance.
Housing charities described the Tory promise "as a small sep in the right direction".
But there were warnings that the plans would hurt social housing provision.
Under the scheme the Government will release cheaper, commercial brownfield land for housebuilding - with properties reserved for first-time buyers aged under 40.
Properties would be exempt from many of the taxes on new homes as well as from obligations to build affordable housing alongside new developments.
The Tories first announced a plan to build 100,000 new homes at their party conference last October.
Prices would be capped at £250,000 outside London or £450,000 in the capital.
Colchester is one of the seats that the Conservatives need to win in less than 70 days' time to ensure a majority in the Commons after the election.
A new poll suggested that the Tories had built a lead on Labour.
The latest national poll carried out by Tory peer Lord Ashcroft put the Conservatives on 34 per cent , Labour 31 per cent , Lib Dem 7 per cent , Ukip 14 per cent, Greens 7 per cent.
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