Yesterday was a most difficult day for the LibDems. They LibDems hit 8% in a poll - their lowest for 20 years - on the eve of the university fees vote. With 11 backbenchers and 17 frontbenchers voting for the policy, with 21 against and 8 abstentions, it looks as though almost three-quarters of Nick Clegg's backbench MPs did not vote for his policy. It was the biggest Liberal rebellion since 1918 and beyond. Is Clegg to end up like Lloyd George, ultimately the victim of a Tory cohabitation?
In the time-honoured fashion of governments who have lost a public argument, Ministers put it all down to difficulties of "communication". But whatever errors they made since May pale into insignificance compared with the hole they dug beforehand.
Like control orders, nuclear power, marriage tax breaks and VAT, tuition fees were always going to be a hot policy potato for LibDems in a Coalition government.
But not like this.
Without the decision to sign THAT pledge, to pose for the photos, to make the now cringeworthy videos saying "it is time for promises to be kept", claiming a unique level of integrity and trustworthiness in politics, this could have been just another u-turn in an everyday tale of Coalition folk. It would not have had such billing as a national Whitehall farce seeming to confirm every negative stereotype that anybody has ever held about politicians.
Yet the mystery has not been satisfactorily explained. How did the LibDems get into such a fine mess?
The orthodox argument is cynicism. The LibDems asked for votes on a pledge they never intended to keep. They wanted the votes of students, secretly knowing they would have to break the promise if they ever had any responsibility. (Nick Clegg still confusingly flips between Panglossian claims that this is the best possible policy in the world we live in, and arguing that the LibDems would have acted differently with a majority).
And the cynicism charge appears strengthened because we know that the LibDem leadership were against their own policy before they campaigned so vocally for it. They didn't believe in it - but once stuck with it by their Federal Policy Committee, they could see how it could be used to maximise votes.
Yet the cynicism charge fails to unwrap the enigma. It offers motive, means and short-term opportunity only sufficient to make the yellows the most inept of Machiavellian schemers. The price must be paid for a Faustian pact. How LibDems now regret the decision to maximise the student vote in May 2010 - and so set back the painstaking work of a generation. It is true that this often involved pitched a curious cocktail of genuine political idealism and anti-political stuntery. As every Tory commentator praises the LibDems for growing up, could it ever be glad confident morning in their appeal for yoof votes again?
So perhaps it was not cynicism, so much as stupidity? Was the LibDem problem of seeking short-term electoral gain, but not thinking beyond the election to the potential fallout? But this doesn't really convince either. Danny Alexander and David Laws and the rest of the LibDem negotiating team are not stupid. Far from it.
So the mystery recurs: how would a party which boasts of having made the most detailed preparations for hung Parliament scenarios not even spot that they were pointing a loaded revolver at their own foot? The idea that they didn't think that a u-turn would be such a big deal seems implausible, as they would hardly have made the campus tour such a major theme of their election campaign if they didn't think it was salient.
LibDems don sackcloth and ashes to tour the studios saying it was a mistake to make a pledge they could not keep - but none has offered a satisfactory account of why they did so.
Here's the only explanation I can see that makes sense of the pledge campaign strategy. Perhaps it could be more cynical still, but it isn't stupid anymore.
The parallel universe where the Clegg pledge worked like a dream
If Nick Clegg and the LibDems were sure that the Tories would win the election then everything about the pledge campaign suddenly makes perfect political sense.
The LibDems may have prepared for a hung Parliament, but they didn't expect to get one. If they did get one, they didn't expect a Coalition, rather than supply and confidence, where the choices might have been different, mass abstention might even have looked reasonable, and where the LibDems could have determined terms from outside government.
Their were two motivations for the pledge.
In the short-term, overestimating Cameron's electoral appeal meant they were scared of being wiped out in a Cameron surge of LibDem-Tory marginals. Enthusing the students would help here.
But they may well have been thinking ahead too. For didn't the NUS campaign position the LibDems quite beautifully if you take a trip with me to a parallel universe where David Cameron did win a Tory majority of 12?
In this world, does anybody believe you would have heard Nick Clegg acknowledge a single point he has made this month? (Though I do rather like to think that Vince Cable may have still made a more grown-up speech, about the way a graduate contribution/tax could work, rather similar to his July speech which the Browne review seems to have ignored).
Cynical, yes. But effective, opportunist opposition politics too.
This issue would give the appearance of outflanking Labour to the left in the universities and on the letters page of The Guardian, yet doing so on an ideal issue to get under the Tories' skins in the affluent south-east and southern marginals where parents and students would see the LibDems on their side.
Take a different turn and Nick Clegg could have been heir to Charles Kennedy after all.
They didn't get good at by-elections for nothing you know.
And there would be no need at all for Nick Clegg lectures telling people the party was not a receptacle for the politics of conscience and protest, when he would be so busy leading the protests himself.
Oh, how commentators like David Aaronovitch would growl cogently at the lack of any LibDem policy honesty or credibility. I rather think Nick Clegg would live with that, presenting himself as "the man who is not scared to take on the Establishment".
Far from burning Clegg in effigy, students would not be able to keep him off a campus again. How LibDems would tweet those pledge pictures around.
So how Clegg would bash Labour still, claiming it was Labour who had sold the Tories the pass for, in this universe, fees would never been necessary at all, as he would revive his now forgotten and rather tribalist realignment claim that his party would vanquish the other lot on the centre-left.
Remember that Nick Clegg's main stated political objective for his party was to double his party's seats in two General Elections. If we were in the parallel universe that Clegg anticipated, the NUS pledge was designed to help to put him on that path, instead of creating a massive risk of the party halving their seats.
So perhaps the LibDem leader bet the house on blue, not when he formed the Coalition, but in gambling on the political world he would live in after the election.
Was the fatal flaw of the Clegg strategy that he over-estimated David Cameron's electoral popularity? To be fair to Clegg, his was a mistake that almost everybody else made too, and continues to make still. Overestimating David Cameron has been a central distorting theme of almost all British political commentary for five years. But it may be Clegg and his party who pay the heaviest price.
Who wins the Thick of It Award for political dignity?
It is surely astonishing that former higher education spokesman Stephen Williams, who has given the most candid account of spending years trying to ditch the old LibDem policy finally ended up abstaining yesterday.
Especially when he had serially denounced the very idea of abstention in the strongest possible terms when castigating his predecessor.
Betrayal: Thousands of students and parents will be wondering what the point is of an MP who can't take a stand on such a major issue. Stephen Williams concluded "Bristol West needs an MP who says what he means and means what he says. All we have at the moment is the embarassing spectacle of an MP stuck on the fence.
He said - and then abstained. You couldn't make it up.
Despite stiff competition over the fees issue, Williams surely takes The Thick of It Award.