Nick Clegg has ruled out taking Liberal Democrats into a coalition which relied on "life support" from the Scottish National Party (SNP) and raised questions over the legitimacy of any government led by a party which had finished second in the General Election.
The Liberal Democrat leader's comments - which appear to be a direct riposte to SNP claims that an "anti-Tory majority" can lock David Cameron out of Downing Street even if he commands the largest group of MPs - will be taken by some Labour activists as an indication that Mr Clegg prefers a renewed alliance with Conservatives if the May 7 election produces an inconclusive result.
They came as SNP First Minister Nicola Sturgeon said she expects her party to wield "enormous influence" over Labour if Ed Miliband's party forms a minority administration.
Mr Miliband has ruled out a formal coalition with the SNP in the event of a hung Parliament and has indicated that he does not favour any form of deal with Ms Sturgeon's party, leading to widespread speculation that he may seek to govern alone or with Lib Dem support in a minority administration.
This could leave him reliant on the votes of SNP MPs, who many expect to side with Labour and against Tories in most divisions, even if their leader does not reach a deal to do so.
Ms Sturgeon told The Times: "I know how minority government works. You have influence - particularly with the Fixed-Term Parliament Act.
"You can change the direction of a government or defeat it on particular issues without the government falling and there being a general election. That gives enormous influence to a smaller party in opposition."
But Mr Clegg's comments raise questions over whether such an arrangement would be viable.
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He told the Financial Times: "I totally rule out any arrangements with the SNP - in the same way I rule out any arrangements with Ukip - because there is no meeting point for me with one party that basically wants to pull our country to bits and another party that wants us to pull out of the EU.
"I would never recommend to the Liberal Democrats that we help establish a government which is basically on a life support system, where Alex Salmond could pull the plug any time he wants. No, no, no."
He argued that any coalition including the party with the second-largest group of MPs in the Commons - which many projections suggest could be Labour - would lack "legitimacy" with voters, who would question the Government's "birthright".
Mr Clegg was taking a break from the campaign trail on Saturday while other election activity was being interrupted by commemorations of the centenary of the Gallipoli operation in the First World War.
Asked about the possibility of doing a deal with the Lib Dems after the election, Ed Miliband said he would "let the people decide".
During a visit to Stevenage, the Labour leader told Sky News: "There are some people who are commentating on what happens after the election.
"I'm concentrating on the issues at this election. That's what I'm going to do for the rest of this campaign because I think there are huge issues on the ballot paper, issues like the National Health Service, issues like the economy we create.
"So, we'll leave it to others to commentate. We are going to go out and fight for what we believe in this election."
After a speech in south London, the Prime Minister said voting for the Liberal Democrats was like voting for Forrest Gump.
"You don't know what you are going to get," the Prime Minister told supporters, drawing on the famous line from the film that life is like a box of chocolates.
He was also asked about comments by Mr Clegg that a coalition formed by the second largest party would lack "legitimacy".
Mr Cameron replied: "Nick Clegg, what can I say? The truth is this, there are now 12 days to go before the General Election.
"The Conservative Party needs to hold the seats it currently holds like this one and win 23 more seats to achieve an overall majority. We can do that, it is within our grasp.
"If we do that, all these problems of coalitions, deals ... will go out the window.
"To those people who are watching in seats where there is a Liberal Democrat candidate .... if you vote for that Liberal Democrat, you could end up with Ed Miliband backed by the SNP.
"An end to the economic plan, an end to jobs, an end to growth, an end to the things that are working for this country."