Donald Macintyre's Sketch: A whole lot of bed swapping going on as David Cameron warns voters

David Cameron was the first to threaten that voters might sleep with one party leader and wake with another. Today, the Tory MP Nigel Adams produced a gruesome, strictly after-the-watershed, variant: “If voters go to bed with Nigel Farage on 7 May, they could wake up not only with [Ed Miliband] but snuggled up next to Alex Salmond.”

This obediently followed Cameron’s assertion – as Scottish Nationalist MPs triumphantly punched the air – that Miliband’s only hope was to “crawl through the gates of Downing Street on the coat tails of the SNP”. This tricky procedure was even harder to visualise because according to Cameron, Miliband would also be in Salmond’s “pocket”. Some coat tails! Presumably tailors in Edinburgh can run up the necessary garment.

But need this happen, given talk about a “grand coalition” to sideline the SNP? Ukip’s Douglas Carswell clearly erred with a tedious immigration question instead of warning Tory voters against “going to bed with Cameron and waking up with Miliband”.

This would have focused minds on future Con-Lab Cabinet making. “I need you in my government, Dave, to save the Union.” “Ha! That’s rich, Ed. Three PMQs ago, you called me ‘useless’ and a ‘bully’.” “Well, you called me ‘weak and despicable’. But needs must. How would you like to be Welsh Secretary?”

Cameron started the abuse while trying to deflect Miliband’s repeated taunts about his refusal to go head-to-head in what the PM had approvingly referred to in 2010 as a “proper television debate”: Miliband: “If he is so confident, why is he chickening out?” Cameron: “He wants to talk about a television programme. I want to talk about the future of the country.” Albeit only on his terms, evidently.

Things had started better. Labour’s Stella Creasy asked a grown-up first question, about US dismay at the prospect of lower UK defence spending: “What will be more important to him in the next parliament: protecting our armed forces or introducing tax cuts?” Cameron waffled about combining economic with national security. Then it was all downhill.