The mistake was promising a referendum in the first place. There never was a sound case, properly rooted in the way we conduct our politics in Britain, for any referendum, ever, on anything to do with the EU – including joining it. In the UK we elect governments to make these decisions; we expect party election manifestos to warn us in advance of the really big ones. We ask Parliament to decide.

And if without warning a government makes such a decision anyway, we can chuck it out. If we hate it intensely enough we require the next government to reverse it, wriggle out of it or renege on it. We don’t do that lightly, but we know we always can. As a world power Albion has a fine and ancient reputation for ratting on solemn undertakings. We can always let our foreign friends down, and we frequently do. That’s democracy.

So there goes the first argument for a referendum on this pesky proposed new European treaty: that it’s “irreversible”. Nothing’s irreversible – or why would some Europhobes be demanding a rerun of the original referendum to join? Whatever the rules may suggest, we can review terms and conditions at any time, though this might involve a review of our membership itself. No referendum will settle anything for all time.

Nor will any objective determination ever be reached as to what is a “constitutional” and what a “nonconstitutional” treaty. This is pure metaphysics. No clear distinction exists. No clear distinction ever could, because the term means so much less than it pretends. Any ambitious measure affecting the process of government will involve a hybrid mix of constitutional with nonconstitutional change.

Wasn’t the giving away of our various colonies a constitutional act? It was certainly a massive ceding of sovereignty. And show me the measures in the proposed European treaty that equal, in their haemorrhaging of “sovereignty”, the loss by the great majority of the British voters of almost all say in the governance of three of the four constituent parts of the Union – for that is what devolution has meant, and the majority, the English, were never asked.

And aren’t defence pacts that may oblige us to go to war at the bidding of foreign partners – such as Britain’s accession to Nato – constitutional? Through much of the Cold War, Nato was a sort of Common Defence Policy.

So of course the proposed European treaty is “constitutional”. But so, in an equally hybrid way, was the Single European Act (signed in 1986), or the Maastricht treaty (1992), or the Amsterdam treaty (1997) or the Nice treaty (2001).

We return, then, to the enormous mistake made by Tony Blair’s Cabinet, a mistake in which Gordon Brown was complicit and whose consequences he now inherits. In 2004 it promised a referendum on the emerging European treaty, on the ground that it contained significant changes, some of them of a constitutional kind. In the 2005 Labour manifesto these promises were repeated. Few can seriously deny that there is something dishonourable in Mr Brown’s argument today: that because the amended treaty, which retains many “constitutional” features, has been shorn of a few more that Britain found objectionable, the promise of a referendum no longer applies. The Blair Cabinet weren’t anyway against constitutional measures in a treaty: they were intending to urge us to vote for a “constitutional” treaty, in the referendum that never happened.

A word, though, of warning to the Tories. David Cameron’s case, now, against Mr Brown may be a double whammy, but one of the punches is a big one, the other less potent, and I suspect many Tory MPs and Eurosceptic commentators have ordered them the wrong way round.

The big punch is that this whole referendum business is just another example of Mr Brown’s cowardice, evasiveness and lack of honour. The example’s potency lies not in how much we care about the issue itself, but the new evidence it provides for an already growing suspicion. Likewise, were we to learn that, on his way into the Commons, Mr Brown had promised a beggar some small change on his way out, but in the event sailed straight past, we would all be talking about it; but you would be wrong to think this meant we cared much about the beggar.

So the punch that is feebler is the issue of the treaty referendum itself. Most people will say “yes” if asked whether they’d like to be consulted; most people do. But how deeply and enduringly angry the public are, or are likely to become, about constitutional implications is an altogether different question, inviting a more ambivalent answer.

The truth is that the arguments about whether a measure is “constitutional” have usually been – and are in the European case – proxy arguments for a debate that is really about something else. Like the distinction between “amateur” and “professional” or between “fertilised embryo” and “unborn child”, a faux expertise is set up to prowl the frontier and argue the toss about words, when what lie at the root of the argument are feelings. The real debate, intense among a minority, is about our continued membership of the EU. But among the electorate at large no sharp, settled or strong opinion about this, one way or the other, is clear to me.

So I’d go on the D-word – dishonour – rather than the C-word – constitution. For it’s my belief that though you can get some of the British angry about constitutional questions for some of the time, and a few of them angry for most of the time, you will never get many of them angry for much of the time. We are not hugely interested in constitutions. That’s why we don’t have one. We tend to drift away from arguments about abstract reasoning.

Abstractions are for lawyers: squalls about words. It is events, facts, accidents, that can really whip public opinion into a storm. If the Europhobes are right (I don’t discount the possibility) then one day one of the hypothetical horrors that Europhobes already dimly see, lurking in the undergrowth of the small print of a draft treaty, will leap out and grab the British people by the throat. European President Giuseppe Bloggs will close Great Ormond Street Hospital. The European Parliament will ban domestic cats. The National Trust will be wound up under European competition laws. And all at once we’ll be talking – shouting – about constitutions, and electing a government committed to renege.

But not until then. Short of that, can an argument about a constitution ever catch fire politically in this country? Well, there is a way that the Conservatives could test that. They could go into the next election with no concrete example of Euro-folly currently enraging the voters, but promising to renege on an already ratified treaty. The possible consequence (which would quickly become the central isisue of the election) would be that our entire membership of the EU would begin to unravel. This would prove a fascinating test. It is not one I’d recommend Mr Cameron to try.

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