THE PUBLICATION of an opinion poll suggesting that UKIP is on the verge of becoming the new "third force" in British politics is just the kind of thing to which political commentators are supposed to over-react. But since Survation's latest poll puts UKIP on 22% of the vote, just behind the Conservatives on 24% some reaction - even of the hasty, excitable kind - is understandable and even necessary.

Then again, after weeks of government turmoil over europe, gay marriage and just about any other subject that's cropped up lately, it would be surprising if Tory poll numbers were not slumping. Equally, the acres of newsprint and hours of television time devoted to Nigel Farage have boosted UKIP's visibility to record-breaking levels. In the circumstances, it would be surprising if UKIP were not enjoying a mini-boom. Nevertheless: 22%. Pop that in your pipe and smoke it, Mr Cameron.

But it is still, at least for the moment, only a boomlet not a proper boom built upon secure foundations. It is worth recalling that political fashions come and go. Much of the Westminster media and political classes appears to have forgotten the lessons of the 2010 general election. In case you too have forgotten, recall that for a couple of weeks it looked as though the Liberal Democrats might win more votes - and possibly more seats - than the Labour party. Britain - or at least those paying attention to the election - was gripped by something called Cleggmania.

My, how long ago that seems.

Politics is not immune - indeed is actually acutely vulnerable to - bubbles. Nick Clegg benefited from a bubble in 2010, now it is Nigel Farage's turn to see his stock inflated by speculators convinced they've hit on the Next Big Thing. We have been here before. Many times. In the early 1980s the Liberal-SDP alliance even briefly touched 50% in the opinion polls. Perhaps you remember David Steel and David Owen sharing the keys to Downing Street.

Despite everything there has been no great realignment of British politics. Or, rather, of English politics. Scotland will not always enjoy an SNP government but it seems improbable that the SNP will wither away even if Alex Salmond does lose next year's independence referendum. The nationalists are ensconced as the leading alternative to Labour and it is hard to imagine circumstances in which they are knocked from that perch.

English politics has proved remarkably stable. With the exception of the United States it is hard to think of a political map that has changed less than England's these past 50 years. Indeed even that under-states the degree of continuity south of the border since, in effect, there has been a great realignment of American politics since the civil rights era. The solid Democratic south has swung to the Republicans as part of a shake-up removing conservatives from the Democratic party and moderates from the Republican party. Everyone is now in the "correct" party.

I digress. The point is that the Tory and Labour parties have proved remarkably successful and even more unusually stable. Realignment has always been around the corner but has never quite arrived.

Perhaps it will be different this time. UKIP will certainly poll strongly at next year's european elections and we should expect it to increase its vote at the 2015 general election too. It is not inconceivable that UKIP could poll 10% in England. That might not produce many - or any - seats but it would also make it hard to imagine David Cameron winning a majority.

If - and it remains a hefty if, not least since Westminster elections are fought under different conditions - UKIP sticks around, proving itself more than just another flavour of the month, then it really does have the potential to change British politics. With the electoral boundaries already helping Labour a strong UKIP showing might well be enough to send Ed Miliband to Downing Street.

But what of Scotland? Alex Salmond is Nigel Farage's keenest admirer. Not because the First Minister likes Mr Farage's politics (he does not) but because the better UKIP does in England the happier Scottish nationalists will be. UKIP is a largely English phenomenon. Though Scots are almost as eurosceptic as English voters the european issue is a lower-order priority north of the border. UKIP's value, then, lies in illustrating the nationalist theme that Scotland and England have distinct political cultures.

And not just distinct cultures but cultures that are growing further and further apart. The SNP line - which is not wholly mistaken - is that Scotland and England are drifting apart anyway and, since this is the case, independence is merely the logical conclusion to this process of estrangement. A UKIP on the march in England but bereft of followers in Scotland is a useful way of illustrating this point.

There is something to this. But only something. Salmond and the SNP should be careful. If UKIP stick around they will complicate another SNP meme. For months now, the SNP and the wider Yes campaign have been arguing that only independence can save Scotland from the horrors of future Tory governments at Westminster. It's independence or five more years of David Cameron. Given the relentlessness with which the nationalists have pursued this line, one has to conclude that they think it is effective.

But it only works if it looks as though the Tories might win the next Westminster election. If, on the other hand, Ed Miliband seems likely to prevail then the game changes and some Labour supporters contemplating voting for independence may drift back into the No camp. In other words, Salmond needs Cameron to remain viable even as he also hopes Nigel Farage can change English politics for good. Almost all of this, of course, is beyond the SNP's control and a reminder that luck will play a larger role in the independence debate than is sometimes assumed.

Farage has been lucky thus far and so, you might argue, has Salmond. But like so much else that can change too.