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.
David Cameron earlier defended Michael Fallon's personal attack on Ed Miliband, but has now said "it's important to be respectful".
Speaking at a PM Direct event at the National Grid training centre in Nottinghamshire, Mr Cameron was asked about the Defence Secretary's claim that the Labour leader "stabbed his brother in the back" over their 2010 leadership contest.
He added that Mr Fallon had been making a key point about security and was right to "raise it in a pretty frank way, as he did".
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?