Jim Murphy’s practiced air of nonchalant bonhomie was coming apart at the seams all over today’s “Good Morning Scotland” (from 2h 10m). Pressed hard by presenter Bill Whiteford, the beleaguered Scottish Labour branch manager spluttered and blustered and interrupted constantly in a desperate attempt to stop Whiteford from even finishing any questions, never mind getting answers to them.

Murphy tried determinedly and repeatedly to punt the hopelessly-discredited line about the biggest party forming the government, on the sole basis that it had always been the case before, seemingly unaware that the election wasn’t being held in the past. He even tried to use the recent catastrophic Ashcroft polls to Labour’s advantage.

You can marvel at the entire nine-minute trainwreck by clicking the link below. But let’s just pull out that one argument and take a closer look at it.

Two minutes into our clip, Murphy says this:

“An opinion poll yesterday showed that Labour and Tory are tied at 272 seats each. Now that’s figuring in the opinion polls in Scotland, which I accept are bloody awful for the Scottish Labour Party and really pretty brilliant for David Cameron, because at 272 vs 272, every single Scot has the future of Britain in their hands, and each and every seat that Labour can win in Scotland gives us a chance of being the biggest party, forming a government [blah blah blah]“

The problem, obviously, is that that’s a shedful of drivel. If that particular poll does indeed turn out to be accurate then the Conservatives have already lost, because it gave the SNP 56 seats in Scotland, and we already know that the SNP won’t under any circumstances help a Tory government into power.

272 Labour plus 272 Tory plus 56 SNP is exactly 600 seats. That leaves only 50 in the 650-seat House Of Commons, which would mean the MOST votes the Tories would be able to muster arithmetically would be 322 – not enough to win a confidence vote and take power with Labour and the SNP opposed.

(And they have no chance of getting even that close anyway, because some of the remaining 50 will be Plaid Cymru, Green, SDLP, absentee Sinn Fein and so on.)

So even in Murphy’s ultimate nightmare scenario, where Scottish Labour are almost completely wiped out, Labour would be the only party that could possibly form a government. With 272 seats and support on a confidence-and-supply basis from 56 SNP MPs, Ed Miliband would be able to command 328 votes – a narrow majority, even without any kind of backing from the Lib Dems or any of the left parties.

Realistically, of course, the SNP aren’t going to get 56 seats. Two-thirds of that figure would be an astonishingly good result for the Nats. But even if they do, it still wouldn’t let the Tories in on the results Murphy cited this morning. Only Labour could win.

It’s a funny definition of “really pretty brilliant for David Cameron”. But with Scottish Labour currently tripping over their own feet left, right and centre as they stumble and stutter and spin and backtrack and U-turn on an almost daily basis, we suppose we’d be a bit befuddled if we were Jim Murphy too