The SNP should focus on using the powers they already have to improve on public services rather than pursuing a "neverendum" and having uncosted policies, Liberal Democrat Jo Swinson has said.
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."
UKIP leader Nigel Farage has accued David Cameron, Ed Miliband and Nick Clegg of "appalling weakness in the face of the aggressive tactics of the SNP", which he says will "threaten the future of the United Kingdom".
Mr Farage was speaking in response to the SNP's election manifesto, which was launched this morning.
Only UKIP will stand up for the interests of English voters and Welsh voters too. Only UKIP says that votes on matters only affecting England should be limited to English MPs at every stage of the legislative process.
The SNP pledges a team of its MPs would deliver a "stronger Scotland" while behaving "responsibly and constructively" for the UK as a whole.Read the full story ›
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.
Nicola Sturgeon was met with laughter when her impassioned answer on what the SNP would do in Westminster went awry.
"I am not going to Westminster," Sturgeon began before correcting herself.
"I am not going to Westminster at all," she said smiling before continuing, "The SNP is not going to Westminster to bring down governments".
Police have said that their search for the family of six who suddenly left their Slough home earlier this month is still a missing persons inquiry, but that they are "deeply concerned" that they seem to be travelling towards Syria.
Deputy Chief Constable John Campbell said: "I think its legitimate for us to hope for the best, but fear for the worst."
He added the force would be "happy to be proved wrong", and urged the family to make contact and put any concerns to rest.
Asif Malik, his partner Sara Kiran, and their four children are thought to have travelled through Budapest on 12 April.
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."
ITV News Deputy Political Editor Chris Ship notes after Nicola Sturgeon reveals the SNP's plan for a UK-wide mansion tax:
Little surprise SNP support a UK wide mansion tax - given most of those mansions are not in Scotland #SNPManifesto
SNP leader Nicola Sturgeon has promised voters in the rest of the UK her party would use its influence after the General Election "responsibly and constructively".
Scotland's First Minister said her party would "not do any deals which would put the Tories into power" if there is a hung parliament after the May 7 vote.
The SNP's manifesto "above all else" seeks to end austerity, with a proposed £180 billion increase in public spending across the UK, she told supporters at its launch.
Sturgeon also restated her party's opposition to Trident, saying her party's MPs would seek to build a "progressive alliance" against the renewal of the nuclear weapons system.