A Labour government at Westminster propped up by the SNP threatens "backdoor independence" for Scotland, according to Conservative MP David Mundell.
Ed Miliband would be a weak prime minister, subject to "every whim of Alex Salmond's ego", the politician told his party's conference.
Labour has refused to rule out a deal with the SNP in the event of a hung parliament in the general election while Nicola Sturgeon has suggested her party could agree to a "confidence and supply" arrangement.
Mr Salmond is standing for election to Westminster in the Gordon constituency.
Mr Mundell said the former first minister would become a "self-styled puppet master" pulling both Mr Miliband and Ms Sturgeon's strings.
Addressing the conference, the MP for Dumfriesshire, Clydesdale & Tweeddale said: "Let's face it - Labour and the two Eds (Miliband and Balls) are a real and present danger who threaten all this country has achieved.
"They would plunge us back into the dark days - our worst nightmare.
"Or so we thought, but just when you thought it was safe to return to the polling booth, enter stage left Alex Salmond, the self-styled puppet master.
"He'll be pulling the strings of both Nicola Sturgeon and Ed Miliband.
"And I say stage left because the dangerous economics of socialism are shared by Miliband and Salmond."
He added: "So, the choice is stark and clear at what will be the most important general election in a generation.
"It is a choice between competence and chaos, David Cameron or Ed Miliband as prime minister and whether Britain keeps going forward or we turn back and throw away the progress we have made and return to wasteful spending, paid for by borrowing and higher taxes."
A Conservative majority is needed, Mr Mundell said, to ensure economic progress is "not jeopardised by an already weak Ed Miliband who is subject to every whim of Alex Salmond's ego".
"With the threat of the chaos of Labour and the scheming manipulative plans of the SNP for backdoor independence this is the moment to step up, to take the fight to those who threaten our country's future," he added.
SNP MP Stewart Hosie said: "Even the Tories are now acknowledging that a strong team of SNP MPs will be able to exert power at Westminster on behalf of the people of Scotland - standing up for Scottish interests and opposing the obsession with austerity of the Westminster establishment.
"The Tories are right to be concerned that a strong SNP vote will stand in the way of their damaging agenda - a poll earlier this month showed that people in Scotland are backing the SNP to keep the Tories out of office."
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