The Conservatives are accusing Ed Miliband of preparing to "stab the United Kingdom in the back", trading away Britain's nuclear deterrent in order to secure power in a backroom deal with the Scottish Nationalists.
Ukip leader Nigel Farage and Green party leader Natalie Bennett have condemned a personal attack on Ed Miliband by the Conservatives.
Defence Secretary Michael Fallon made reference to the Labour leadership contest between Mr Miliband and his brother David, and suggested he would 'stab the UK in the back' over Trident.
Mr Farage said he fears the election campaign is becoming "an American, negative, shouting match".
Speaking in Kent, he said: "The attack on Ed Miliband was very, very personal, calling him a backstabber.
"I just fear that we have an election campaign that is turning into an American, negative, shouting match between two parties, and I don't think the public like it.
"I certainly don't."
During a visit to Norwich, Ms Bennett said: "A very important issue has been raised in the debate about Trident today but the way in which it was raised is deeply damaging.
"This kind of personalised attack is the kind of Punch and Judy politics that is really damaging our political fabric."
The main political parties are wrangling over whether the UK should spend billions renewing its nuclear deterrent.
If a difference of opinion on defence didn't get in the way of the current coalition - why would it be different for Labour and the SNP?