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Nicola Sturgeon has said the SNP would help make Ed Miliband prime minister if the Conservatives failed to win a majority in the general election.
During his response in an STV debate, Scottish Labour leader Jim Murphy said his party did not need any SNP help.
Scottish Conservative leader Ruth Davidson ruled out any deal with the SNP.
And Scottish Liberal Democrat leader Willie Rennie said it would be unreasonable to put the SNP in charge.
In the first televised Scottish leaders' debate, Mr Murphy asked the first minister: "Nicola, do you want Ed Miliband to be prime minister?"
She told him: "I' don't want David Cameron to be prime minister, I'm offering to help make Ed Miliband prime minister.
"I've said to Ed Miliband and I'll say to Jim Murphy this evening, that if there is an anti-Tory majority in the House of Commons after the election, even if the Tories are the biggest party we will work with Labour to keep David Cameron out of Downing Street."
Mr Murphy insisted his party did not need "help" from the Nationalists to oust David Cameron from 10 Downing Street.
Spending increase of 0.5% a year, enabling £140bn extra investment
Annual UK target of 100,000 affordable homes
Increase in minimum wage to £8.70 by 2020
Restore the 50p top income tax rate for those earning more than £150,000; introduce a mansion tax and a bankers' bonus tax
Build an alliance against the renewal of Trident
Retain the triple lock on pensions and protect the winter fuel allowance
He said: "Nicola, we don't need your help. What we need is people north and south of the border, people in Scotland, people in England and people in across Wales coming together to kick out an out-of-touch government."
He said any seat Labour lost in Scotland would increase the chances of Mr Cameron being prime minster.
He stressed that key in the election was ensuring that "working people do better" after the ballot.
While Labour has seen support fall away in Scotland, he added: "Labour has changed, and we will continue to change."
Ms Davidson ruled out any deal with the SNP and challenged Ms Sturgeon on her plans to support a minority Labour administration.
She said: "Why is Nicola running around saying 'Labour is rubbish, vote for me so I can put them in office'?"
Mr Rennie said it would not be reasonable to put the SNP in charge as they were in favour of breaking up the United Kingdom.
Ms Davidson insisted her party had tried to rebuild the economy after the recession with some of the most vulnerable in mind.
"That's why we're trying to get people back into jobs," she said.
"That's also why we've taken people at the lowest end of the wage spectrum out of taxation altogether, so they keep more of the money they get."
Mr Rennie said his party had helped get the economy back on track.
He also highlighted the increase in the income tax threshold under the Conservative-Liberal Democrat coalition government.
He said: "What we've done, with one of the biggest recessions this country has ever faced, is we've got the economy back on track and we've done it fairly."
He said he wanted to maintain the "progress" the Liberal Democrats had made in government in the next five years.
"I don't want to veer off to the left or the right like the others propose," he said. "I want to keep on that path and keep that recovery going.
"What we need to do to ensure that we do that is invest in the NHS and make sure we balance the books."
During the two-hour debate, Ms Sturgeon said she respected the result of the independence referendum last year and insisted the Westminster election was "not a re-run of the referendum campaign".
She said a vote for the SNP meant a loud voice for Scotland at Westminster.
Former SNP leader and first minister Alex Salmond had said a further referendum was off the agenda for a generation.
Nicola Sturgeon said it was a decision for the people of Scotland, not politicians. "I can't impose a referendum," she said.
She also said her party would "help deliver progressive change", adding: "We propose a real alternative to the pain of austerity, an end to the Bedroom Tax, a higher minimum wage and protection for our NHS and valued public services."
On Wednesday, Mr Murphy, Ms Sturgeon, Ms Davidson and Mr Rennie will take part in a BBC Scotland debate along with Patrick Harvie of the Scottish Greens and UKIP's David Coburn.
The debate will be hosted by the BBC's James Cook from Aberdeen and will be shown on BBC One Scotland and online from 21:00.