Grand financial crises aside, Gordon Brown rarely does much by accident.

Where the independence argument is concerned, he has seemed to choose his ground carefully. His, we heard, would be a referendum campaign for the United Kingdom, but not one involving any party governing the UK that didn't happen to be Labour.

Better Together could live with that, it appeared. Equally, the campaign to prevent independence had precious little choice. Who tells a former prime minister how he must and must not campaign? Who tells the man who once reputedly (meaning mythically) "ran Labour in Scotland" what was best for a party in the fight of its life?

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In fairness, Mr Brown brought more than that to the argument. He did much, in years past, to shape arguments for devolution. As Chancellor and Prime Minister, he struggled repeatedly to express what it could, should, or might mean to be British. He has thought more about the issues than most of those who imagine Scotland can be fixed with the usual banal mixture of spin and spite.

Until this week there was agreement, in any case, over Mr Brown's value to the Union. If anyone could prevent traditional Labour supporters, so called, from being swayed by the arguments for Yes, he was the man. The Scottish "working class vote" that did not waver throughout his Downing Street travails would be more receptive to the Better Together message delivered by Gordon than by any conceivable alternative, not least if he kept an ostentatious distance between himself and Tory-funded Better Together.

It felt like a kind of truce, but until this week that only mattered to Scottish Labour folk and a few journalists. Even its enthusiasts have never claimed that Better Together is more than an uneasy alliance. Even when Tories persisted in sniping at Alistair Darling, the campaign's chairman, all concerned maintained the fiction that differences had been well understood and absorbed for the greater good of the UK. They kept this up even while failing to agree on a future for that UK, far less for Scotland.

This week, Mr Brown chucked the script in the bin. Why was that? Should David Cameron engage in a public debate with Alex Salmond? Yes, said Gordon in interviews. Has the Better Together campaign been misconceived? Mr Brown detected some crucial flaws, whether in the choice of opposing arguments, or in the woefully patronising tone of people who think it "humorous" to tell Scots how many chips a little plastic person could buy for £1400. The big clunking fist has been clenched.

You can only speculate as to reasons, but take care to note reactions from Mr Brown's side. In recent interviews, neither Johann Lamont, Lord John Reid nor Mr Darling have exactly enthused over their former leader's intervention. Scottish Labour has seemed a little less than united. Did the former PM have his strategy proposals rejected, then take the huff and press on regardless with that old, familiar hubris? Is he attempting to assert control over a campaign in trouble? Is he oblivious, vindictive, or cunning?

"Cunning" would be the consolation prize for Unionists. In this version of the tale, Mr Brown is performing the kind of trick that put himself and Tony Blair into government. Seeming to pick a fight with your own side in order to secure the voters you want is one of the tricks new Labour stole from America's Democrats. In our version, the purpose of Mr Brown taking a pop at Mr Cameron, "London", and a Better Together campaign he disdains is to reassure Labour-inclined Scots.

You could add a wrinkle. As Mr Darling has written, the relationship between a Chancellor and his Prime Minister was not easy. Words were exchanged, to put it no higher, between two old friends and colleagues. We were supposed to believe that a rapprochement had been achieved for the sake of the Union. This week we know that Mr Brown has made things difficult for Mr Darling, but the former seems not to care if the latter is aggrieved. Who keeps the scores before they are settled?

Britain's most recent Labour Prime Minister is not done. In an article published yesterday by the Guardian, he made several statements that, if accepted, would take the Better Together campaign to the outer edges of federalism and leave some British patriots wondering what, exactly, they have been fighting for. That big beast from Fife turns out to be a constitutional radical.

Mr Brown writes: "Westminster's claim to undivided authority over the country? Dead and buried. The constitutional fiction that parliament, or the Queen in parliament, rather than the people, are sovereign? Gone for ever."

Forget "entrenchment" of Holyrood, or abstruse arguments over the Declaration of Arbroath: Her Majesty's former Prime Minister is saying that an entire political tradition, Crown included, is null and void.

While attempting to give pragmatic economic reasons for perpetuating the UK, while attempting to appeal to a desire for security, while seeking to instil fear of the unknown rather than the grisly known, Mr Brown strips the Union to a skeleton: "a covenant between nations". He might as well - though he does not - call for commissioners to be summoned to negotiate a treaty between sovereign peoples. The former Prime Minister thinks, in fact, that this is already taking place.

Better Together are never delighted by independent thinking, but that's the risk you run if you let loose big beasts who never forget a slight. For those who admire Mr Brown, the more important question might be, "Will it work?" Both the No campaign and the movement for a Scottish future understand that everything now hangs on winning the undecided, especially among the Scottish working class, above all among those for whom voting isn't a habit. Is Gordon still a voice in the land?

I think not. This is not because his thoughts are less than interesting, rather because the argument has moved into realms that conventional politicians - and journalists - scarcely understand. The latest TNS survey says an extraordinary 28% still count themselves as "don't know" in this debate. My guess, just that, is that a crucial portion of the electorate refuse to have any truck with pollsters, or to make a choice, until, on the day, they must. These undecideds are implacable.

Will they be swayed because a former favourite son sows confusion among alleged allies? That doesn't seem likely. Will they respond tribally to a Scottish Labour party at odds with itself? The question is patronising and misses the point of a unique contest. Mr Brown's intervention will be noted, as will its effects on the campaign he is supposed to be espousing. What's your reaction when a big beast goes rogue? That's one for the privacy of the voting booth.

But the former PM does Scotland one singular favour. His actions, like his words, concede that a large part of the battle is already over. "Scotland," as he says, "has already changed Britain for ever." There is no going back. So how should Scotland go forward? By claiming every right, or only those few rights deemed appropriate by big beasts in a confused, fading Union?