A Conservative poster that depicts Ed Miliband in Alex Salmond's pocket shows Westminster has been "totally rocked" by the SNP, a Scottish government minister has said.
Humza Yousaf also predicted the poster would increase support for the SNP.
The Tories unveiled the poster after calling on Labour to explicitly rule out a post-election deal with the SNP.
Labour's Shadow Chancellor Ed Balls has said coalition with the SNP is "not part of our plans".
Opinion polls have consistently suggested the SNP is on course to win the majority of Scotland's 59 seats in the general election on 7 May.
This has led to speculation that the party could hold the balance of power in the event of a hung parliament, which many pollsters believe is the most likely outcome.
'Having a voice'
On Saturday, Prime Minster David Cameron called on Labour leader Mr Miliband to rule out a deal with the SNP "if he cares about this country".
Mr Cameron tweeted an image of the new Conservative poster on Monday alongside the caption: "Alex Salmond with Ed Miliband in his pocket. The frightening prospect we must avoid."
In response, Mr Yousaf, the Scottish government's international development minister, said: "It is clear that the establishment is utterly rocked at the prospect of Scotland having a voice.
"The more and more the prime minister talks about the SNP and talks down the SNP it will only increase our support in Scotland."
The SNP has never said it would be willing to enter a coalition government if no single party commands a majority in the House of Commons.
The party has said it would not do any kind of deal with the Conservatives after the election, but has not ruled out the possibility of supporting a minority Labour government on an issue-by-issue basis, an arrangement known as "confidence and supply".
Asked on Monday whether he could categorically rule out a deal with the SNP, Mr Balls said: "We have had for the last 48 hours a scare from the Conservatives about the SNP and coalitions. It's a complete nonsense argument. We want a majority and that's what we are fighting for."
Mr Balls added: "The SNP have said they don't want a coalition. It's not part of our plans. We don't want one, we don't need one, we're not after one.
"No large party in the last 100 years - Labour or Conservative - has ever fought a general election on the basis they wanted a coalition or deal with a small party. It's the last thing we want. What we want is a majority Labour government."
Mr Balls was speaking after delivering a speech in London in which he accused the Conservatives of planning "extreme" post-election public spending cuts of £70bn as he unveiled Labour's analysis of how Conservative plans would affect non-protected Whitehall departments.
Responding to Mr Balls' speech, Financial Secretary to the Treasury David Gauke said: "Ed Balls has today confirmed Labour's policy of billions of pounds of tax rises, more borrowing and more debt - which in reality would be even higher with an Ed Miliband government in the pocket of Alex Salmond and the SNP.
"There is a clear choice at this election: sticking with the competence and stability of David Cameron and the Conservatives' long-term economic plan that's securing a better future for Britain - the deficit has been halved, there are 1.85 million more people with the security of a regular wage and the economy is recovering from Labour's Great Recession.
"Or abandoning that plan for the SNP and Labour with hardworking taxpayers paying the price for the economic chaos that would result."
Liberal Democrat leader Nick Clegg told a policy launch in east London: "Our view is that, in the same way we would not countenance putting UKIP in charge of the EU, we would not countenance putting the SNP in charge of a country that they want to rip apart. It's as simple as that."