Ed Miliband has repeatedly refused to rule out a post-election deal with the Scottish National Party, as David Cameron sought to ramp up fears about higher borrowing and taxes if Nicola Sturgeon props up a Labour government.
The Opposition leader insisted his message to Scotland's First Minister was "thanks, but no thanks" but declined to give a guarantee that he would not turn to the nationalists for support on a vote-by-vote basis if the outcome of the May 7 General Election is inconclusive.
Mr Miliband insisted that anyone watching last night's televised leaders' debate will have seen "fundamental differences" between Labour and the SNP on the economy, national security and the break-up of the UK.
He said how other parties vote on a Labour Queen's Speech is a "frankly a matter for them" when questioned on possible future pacts.
During a visit to Lincoln he said: "My message to Nicola Sturgeon is thanks but no thanks. Vote Labour for a Labour government."
He added: "I think lots and lots of people are jumping way ahead here. I'm working for a majority Labour government. I believe we can win a majority Labour government. Let's actually see what happens on May 7."
Mr Cameron warned that a Labour government propped up by the SNP would put Britain's "jobs miracle" at risk, as he hailed record employment figures showing more than 31 million people in work.
In a Twitter message to voters, the PM said: "Ed Miliband won't rule out a vote-by-vote deal with the SNP so he can be PM. It would mean more borrowing and more taxes and you would pay."
Unveiling the Tories' jobs manifesto on a campaign visit to the West Midlands, Mr Cameron said a Miliband administration would put a million more people on dole queues if it repeated the mistakes of the last Labour government.
Pressed on the influence Ms Sturgeon could potentially wield over a minority Labour regime, Mr Miliband said: "It's not going to happen and let me explain why - I ask people to judge me on what I said last night.
"I will never compromise our national security, I will never compromise our commitment to fiscal responsibility, I will never compromise on the nature of our United Kingdom.
"Frankly, the first budget of the Labour government is going to be written by the Labour government. It is not going to be written by Nicola Sturgeon or Alex Salmond or anybody else in the SNP. I couldn't be clearer than that."
Ms Sturgeon said that Mr Cameron's only route back to power if the election produces "an anti-Tory majority" would be "if Ed Miliband stands back and lets him back into Downing Street".
Speaking in her home town of Irvine in Ayrshire, as she prepared for Monday's SNP manifesto launch, she said: "If Ed Miliband and Labour do that, they will not be forgiven, certainly in Scotland and I suspect in large parts of the UK as well."
Mr Miliband was judged the "winner" of Thursday's five-way televised debate and viewers taking part in an instant poll favoured him as prime minister over Mr Cameron - who did not take part - by a margin of 45% to 40%.
Mr Cameron rejected a direct challenge from the Labour leader to face him in a head-to-head TV showdown before voters go to the polls.
But Nick Clegg - who was in the Scottish seat of Gordon trying to fend off ex-SNP leader Alex Salmond's bid to return to Westminster - said he was prepared to take on Mr Miliband, after being excluded from the "challengers' debate".
In a poll of 1,013 viewers conducted by Survation for the Daily Mirror, Mr Miliband came out on top in last night's debate, with 35% judging him the winner, narrowly ahead of Nicola Sturgeon on 31%. Ukip leader Nigel Farage was third on 27% followed by the Greens' Natalie Bennett on 5% and Plaid Cymru leader Leanne Wood on 2%.
Among Scottish viewers, Ms Sturgeon was judged to have performed best by a massive margin of 68%, to Mr Miliband's 17%.
Mr Farage told the BBC News Channel: "It's completely clear that the only way that Ed Miliband can become prime minister is with the SNP's support.
"I am very worried about the Scottish tail wagging the English dog. I'm very worried as to what that means for English taxpayers. I'm very worried about what it means when it comes to immigration and all of these issues.
"I would just say, if you are a Labour voter who's worried about the kind of issues I'm worried about, be very, very careful before you vote."
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