Independence is, we are often told, the Scottish National party's raison d'être, its "big idea", yet you would be hard-pushed to notice it in the ongoing Holyrood election campaign. Nevertheless, the Scottish Labour party has gone on the attack, warning of the "disaster" that separation would bring. But the truth is that independence has been politically ringfenced, successfully separated from the electoral fortunes of the SNP itself.

During the SNP's first term in government in Scotland, its promise to hold a referendum on independence within the lifetime of that parliament quickly became a millstone round Alex Salmond's neck. Lacking an overall majority and with the Liberal Democrats unwilling to play ball, the first minister had to buy time with "national conversations", draft white papers, white papers and so on. Finally, at the end of last year, he announced that a referendum bill would not, after all, be laid before MSPs.

Yet love it or loathe it, the independence question is one that the SNP leadership eventually has to address. At present, Salmond's line is that, if his party is re-elected, most likely as another minority administration with tacit Tory support, then there "will be a moral and democratic imperative" upon the opposition (or "unionist") parties to support a referendum in parliament.

Except it won't be that easy, and Salmond knows it. Ironically, the SNP has become a victim of its own success. Four years ago, Scots voters wanted a government that would "stand up for Scotland"; now they've got it, they aren't particularly interested in going any further. As recent academic research has demonstrated, public support for independence has, at best, flatlined over the past three years at about 30%.

Paradoxically, the more electorally successful the SNP becomes, the harder it becomes to sell independence. Indeed, the SNP has been sold in precisely those terms: "Vote for us, we'll do things better than Labour and don't worry, we're not really serious about independence." This line can only hold for so long. Sooner or later the SNP will have to bite the bullet and kiss goodbye to its "big idea".

I suspect some senior Nationalists, including Salmond, see this, but saying so is another matter. There were tentative moves about a year ago, when the first minister admitted to the Times that "the centre of gravity in Scottish politics currently is clearly not independence". He quickly backtracked, but the cat was out of the bag. Salmond then gradually threw in his lot with the campaign for "full fiscal autonomy", which has the benefit (unlike independence) of enjoying broad political and public support.

The trouble is this. Independence for many Nationalists is an article of faith. Indeed, it's a bit like belief in God and the Church of England: most Anglicans do not hesitate in affirming God's existence, yet they are divided on what "God" actually is and an increasing number of clergy don't actually believe in him at all. Just as these fault lines could tear apart the established church, repudiating independence could revive the weary old battle between "gradualists" and "fundamentalists" within the SNP.

Salmond, however, has never been in the business of tearing his party apart, so simply won't go there. Yet there is much to be gained by a more realistic constitutional strategy. Jordi Pujol, the president of Catalonia from 1980 to 2003, is an obvious role model. During that time he significantly increased Catalan autonomy within Spain, advocating federalism rather than full separation.

It is a truth seldom acknowledged that, politically, independence – even for the SNP – is a non-starter. The party has always liked to see itself as a quasi-spiritual "national movement", a broad church, in which case it's time for more liberal sections of the congregation to speak up. At its inception, the SNP believed in "imperial federation" – dominion status within the British empire. Perhaps the national movement is about to come full circle.