Former Liberal leader Lord Steel says Sir John Major does not understand Scotland.
He told the BBC Radio 4 Today programme:
The Tories are really doing the SNP a favour by bringing them to the centre stage of the campaign.
I think the consequence is we will end up with a minority government. Ed Miliband has made it clear he does not want any deal with the SNP.
With 16 days to polling on May 7, Sir John will say in a speech in the Midlands that a Labour-SNP administration would lead to "weak and unstable" government and wreck Britain's economic recovery.
Sir John will give a speech later today saying a Labour-SNP government would be "weak and unstable" and wreck Britain's economic recovery.Read the full story ›
David Cameron told ITV News that he British voters needed to realise the "frightening" prospect of a potential Labour-SNP government.
"I think it is very important everyone understands what is at stake at this election - the stakes frankly have just got much, much higher and the prospects of a Ed Miliband-SNP government I think frankly are very frightening," the Prime Minister said.
ITV News' James Mates reports:
Nick Clegg has admitted there would be a "greater wariness" from the Liberal Democrats about entering a coalition for a second time.
Mr Clegg revealed he had "words" with party candidate Andrew George, who reportedly said a new coalition between the Tories and the Lib Dems was not going to happen.
Speaking to reporters before arriving in Cornwall, Mr Clegg acknowledged there would be concerns about another post-election agreement with David Cameron's party.
He said: "I think if we were to enter into coalition again of course there would be much greater wariness. Of course there would be. It would be bizarre if people weren't wary after the loss of MEPs and councillors and so on. Of course people are wary.
"But wariness is one thing. Imposing some pre-emptive fatwa on any coalition arrangements before people have their say is entirely another and we're not doing that."
The Conservatives lead Labour by four points according to the latest poll issued from Lord Ashcroft.
The poll, conducted over the weekend, said the Tories are up one point since last week while Labour had dropped three.
Ukip were unchanged at 13%, the Liberal Democrats up one point at 10%, the Greens down two at 4% and the SNP up two points at 6%.
Lord Ashcroft said the poll also showed a higher proportion of voters said they were moving towards the three established parties, with fewer moving away.
The SNP has only one aim and that is to break up the United Kingdom, David Cameron has said.
Speaking in Crewe as the SNP launched their manifesto, Mr Cameron said: the party could make strong arguments and "produce nice looking documents" but ultimately they did not want the UK "to exist."
David Cameron was today accused of trying to frighten voters against voting for the SNP in the General Election.
ITV News' James Mates asked the Prime Minister whether he would have to change his strategy as the polls show voters are not frightening so easily.
A coalition between Labour and the SNP would be a "match made in hell" for the UK economy, David Cameron has warned.
Mr Cameron said Ed Miliband's party was "facing wipeout" in Scotland and the only way he could end up in Downing Street was "on the coat tails of the SNP".
He appealed to voters to vote for the Conservatives to avoid a partnership between "Labour and the SNP - the party that would bankrupt our country and the party that would break up our country".
Speaking in Crewe, Mr Cameron said: "Make no mistake, if Labour and the SNP get into power, you are going to see an alliance between a party that wants to spend, borrow and tax more, with a party that wants to spend, borrow and tax even more.
"It might be a match made in heaven for them but it is a match made in hell for the British economy.
"You would see our deficit climbing up again, spending on welfare soaring again, businesses crushed again, jobs lost again. With Labour and the SNP, our economy will head into ruin again. And who will pay? You will pay - in higher taxes."
SNP plans to abolish air passenger duty in Scotland could impact on airports in the rest of the country, David Cameron said.
Speaking in Crewe, Mr Cameron said: "The SNP government in Scotland is committed to using its new powers to cut and eventually abolish air passenger duty for flights from Scottish airports
"That could distort competition and see business drawn north of the border with a huge impact on airports in the rest of our country so we're reviewing the way air passenger duty works to make sure other cities don't loose out".
Announcing the Conservatives 'Carlisle Plan', Mr Cameron said a Conservative government would conduct an annual review of the impact of all devolved policies on England, Wales and Northern Ireland.
"This is not about a UK government stopping the Scottish government from using its powers as it sees fit to do things differently, he said.
"This is about making sure we understand the impact that devolution is having and making sure the rest of the country never unwittingly loses out."
The Conservatives have accused Labour of attempting to "weaponise the NHS" as the party prepares to launch a week of intensive election campaigning on the health service.
A Tory spokesman said Ed Miliband's credibility on health "is in tatters because he refuses to fund the £8 billion the NHS needs."
"By building a stronger economy, we have protected and improved the NHS with 9,500 more doctors, 6,900 more nurses, and 1.3 million more life-saving operations every year," the spokesman said.
"There is only one threat to the future of our NHS and that is the economic chaos of an Ed Miliband-SNP government."