Just when you think you might have heard them all, along comes a brand of sophistry so ingenious it deserves a wee gold star for cheek.

So you thought the local elections left a scene of unrelieved carnage and despair for Nick Clegg, Deputy Prime Minister? Not a bit of it.

To hear it told last week by Willie Rennie, Scottish leader, among several others, we were all too busy counting the fallen to grasp a higher purpose, or understand what has been done for our sakes. As it turns out, greater love hath no LibDem than to lay down his party for the sake of the country.

Nick "Sad" Clegg means that we take this seriously. He's not in coalition for himself: the suggestion is unworthy. Nor is he in coalition for the pursuit of party interest. The AV referendum, shortly to be followed by Lords reform, rendered that notion untenable. Besides, at the current rate of attrition, there will be no LibDems left standing with an interest to pursue.

Mr Clegg, relaunching a lead balloon with David Cameron yesterday, is set on a more glorious course. First, he and his Samaritans are engaged in the tough, necessary work of sedating the Tories to spare us "excesses". This overlooks the tiny, historical detail that there would be no Conservative ministers wrecking the NHS in England – something else the LibDems somehow failed to prevent – or stealing from the disabled in Scotland, had Mr Clegg resisted Mr Cameron's overtures.

Then there's that other noble mission: saving the economy. In Nick's sad tale his party are paying a high price for hard choices. As David Laws, the former not-wholly-disgraced Treasury Chief Secretary was telling broadcasters yesterday, the purpose of the Coalition is to sort out the deficit and undo Labour's (alleged) mess in the face of "headwinds". It's a dirty job, but the LibDems, donning the yellow Marigolds of purity, are staunch.

Another detail: this happens not to be true. The French have kicked out Nicolas Sarkozy despite his boast that at least he spared his country recession. Britain is on its second dose of the purgative, all prospects of growth postponed, all hopes that unemployment soon will peak, deferred. LibDem claims are as comical as Mr Clegg's Lone Ranger impersonation. He mislaid his mask, for one thing.

Mr Cameron has his own problems, it is true. Some of his backbenchers have been talking – how these small details accumulate – as though they actually won the last election, and as though their retainers can be ignored. Their actual meaning is twofold. They want to complete the scorched-earth policy of "reform" while time remains. Then they want to taunt Mr Clegg with a question: what does he dare to do about it?

That's a good question. It almost resembles a riddle. One positive answer from three might have earned the students' friend a spell in political rehab. But if you have not impeded the Tories, failed to save the economy and consigned your party to perdition – the Clegg family will not be holidaying in Edinburgh, I think – the riddle becomes deeply mysterious. Why does "Sad" persist?

Should he ask, George Osborne will tell him that the economy will not be useful for electioneering purposes by 2015. In fact, the Chancellor has already set Danny Alexander, his Treasury intern, to preparing the ground for another 5% of departmental spending cuts, on the immaculately logical grounds that the last lot only made matters worse. Mr Clegg will find no succour there.

Nor, as the local elections demonstrated, will he find relief in the hope that absent-minded voters might just get over it. In large parts of the UK, notably in Scotland, the LibDem leader has managed the astonishing trick of reducing his party to the condition of the Tories: not forgotten, not forgiven. Mr Clegg's local troops took an English walloping last year, and a British hiding this time. At this rate, they will be punished for a generation. At this rate, they will wind up resembling their Liberal forebears in 1957: five MPs, one taxi.

Presumably, Mr Clegg calculates that he can call the Tory bluff. He might be stuck with the Coalition because he dare not risk the inevitable General Election – since no-one believes the "fixed-term parliament" stuff – but how do they fancy their chances? They've seen what happened to Mr Sarkozy and his desperate appeals to right-wing opinion. They've seen the local election results. These parties are roped together, the footholds crumbling.

At this point, the non-aligned would probably ask a question: what does any of that have to do with the national interest of which we hear so much? Bluntly, is this any way to run a country? Is it even an operation capable of forestalling – not that I'm supplying suggestions – Alex Salmond's push for Scottish independence?

If Clegg wants to salvage his party he must walk, and walk now. It's not for me to save the Liberal Democrats from themselves: after the last couple of years, I'd supply the whisky and load the revolver. Dispassionately, though, there is clear evidence that if Westminster's vaunted third force does not extricate itself from the morass, the heirs to Liberal tradition will be sucked under.

Mr Clegg could, if wise – but let's not push our luck – make a virtue of dissolving the Coalition. There are plenty of Tory choices he could, or should, find intolerable. He could make something or other an issue of principle, look doleful, and invite Ed Miliband round for coffee. The ensuing election would be miserable for the LibDems, but it would at least present a route back to self-respect.

Judging by the leaks ahead of today's Queen's Speech, there are no signs of such a thing happening. Instead, Mr Clegg's faction wish only to "differentiate" themselves, just a bit, from their symbionts. So it falls to LibDem activists, such as remain, to ask a simple question. Why is their party being destroyed for the sake of a Coalition that has failed, even on its own terms?

Could it be that the only person still saying: "I'm with Nick" is named Clegg?