UK defence and Trident: Penny Mordaunt and Nick Harvey
Britain and the Soviet Union "detargeted each other 25 years ago" and renewing Trident did not make sense at this stage, said a former Lib Dem defence minister.
Nick Harvey said the public would question the cost of a Cold War-scale deterrent in the current economic and political climate.
But Conservative MP Penny Mordaunt said the "best value and most effective deterrent is what we have currently got", and described the Liberal Democrat position as a "complete mess".
She said Russia was talking about a "first strike" against US missile bases in Poland last year, and future threats to the UK were unknown.
They debated defence policy after Lib Dems suggested reducing the number of Vanguard submarines from four to three.
Watch more clips of films and interviews from 2013; Watch Sunday Interviews with Andrew Neil; Watch our soapbox films; Watch full programmes from the last seven days via BBC iPlayer; Social networks: 'like' us on Facebook page or 'follow' us on Twitter