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SNP leader Nicola Sturgeon has said she would prop up a Labour government, even if the Tories finished the election as the largest party by up to 40 seats.
She told the BBC's Newsnight that "if [the Conservatives] can't command a majority, they can't be a government".
Ms Sturgeon has repeatedly said she would work with Labour to "get rid of the Tories" if the SNP had enough MPs.
Conservative leader David Cameron has called that prospect "frightening" and said it would lead to "economic ruin".
While the SNP had only six MPs at Westminster in the last Parliament, recent polls suggest they could win the majority of Scotland's 59 seats.
That could leave the party holding the balance of power if the election resulted in a hung parliament.
Analysis: Chris Mason, Political Correspondent
On the face of it, the SNP leader is only stating the obvious. She has said before that her party would never work with the Conservatives, but would be willing to work with Labour.
So, if Labour's seats plus the SNP's seats add up to a majority, a deal can be done. But what Nicola Sturgeon's remarks throw up is an issue that could, within a fortnight, prove the biggest of all: Legitimacy.
If the Tories were to win "10, 20, 30, 40 seats" more than Labour, would a Labour and SNP arrangement to form a government be legitimate in the eyes of the electorate?
Some would say the answer to that was no. But, the SNP argues all that matters is what Ms Sturgeon calls the "basic rule of how governments are formed."
Or to borrow a line the Chancellor George Osborne likes to use, the first rule of politics is knowing how to count.
In an interview with Newsnight's Laura Kuenssberg, Ms Sturgeon repeated her offer to back a minority Labour government on an issue-by-issue basis.
"Even if the Tories are the largest party, if there is an anti-Tory majority, my offer to Labour is to work together to keep the Tories out," Scotland's first minister said.
When pressed on whether that would apply even if Labour had secured "10, 20, 30, 40" fewer seats than Mr Cameron's party, she replied: "Governments in the House of Commons are about who can command a majority.
"If there is an anti-Tory majority, yes... we would work with Labour to stop the Tories getting into Downing Street."
Responding to the comments, Liberal Democrat leader Nick Clegg said Ms Sturgeon was going to "ignore the will of the people", if she chose to support the smaller of the two main parties at Westminster.
He added: "She's going to ignore how people vote and she's just going to insist on her personal preference."