WHAT a difference a year makes.

In August last year the latest in a series of polls from TNS-BMRB on attitudes to Scottish independence produced a dramatic result. The gap between those in favour and against independence had shrunk to just one percentage point. Though nearly one in four remained undecided, the pro and anti-independence options were neck and neck. Ergo, the SNP had only to persuade around half of those "don't knows" to back independence and they would be home and dry.

Today's poll, published exclusively in The Herald, tells a very different story. The gap has ballooned to 20%, with half of those questioned rejecting outright independence, while only 30% support such a change. This is the widest margin of support for remaining in the UK since prior to 2007 when the SNP formed their first administration at Holyrood.

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Though the poll was conducted in the immediate aftermath of the launch of the Better Together campaign, the result appears to be more than a temporary phenomenon, as the trend against full independence was already discernable in January.

What has happened to change so many minds? One interpretation is that the more closely voters study the case for political separation, the less attractive it appears. However, there are other factors that could be nudging Scottish voters away from the independence option. The two events massively dominating the 2012 calendar are the Queen's Diamond Jubilee and the London Olympics, both of which emphasise Britishness. By contrast 2014 (the intended date of the referendum) coincides with the 700th anniversary of the Battle of Bannockburn, when Robert the Bruce and his Scottish army sent Edward II and the English "homeward to think again".

Another factor that needs to be weighed is the marked deterioration in the economic outlook. Except during revolutions, constitutional change tends to coincide with periods of stability, when voters are more inclined to take risks. Voters react to uncertainty by sticking with what they are used to on the basis of the instinct depicted by Hilaire Belloc as "always keep a-hold of Nurse, for fear of finding something worse".

This result piles pressure on Alex Salmond to consider putting a second question on the ballot, offering the third way of Devo Max. A second question in today's poll, offering voters that option won support from 37% of those questions, compared with 29% for the status quo and just 23% for independence. Among 18 to 34-year-olds, traditionally thought to be more positive about independence, support for it drops to just 20% under these circumstances and among women to just 19%.

The downside for the Yes campaign is that a second question would clearly split the nationalist vote, as well as the pro-UK one. Also, the Prime Minister has made it clear that he will not sanction a legally binding poll with two questions and an advisory poll could prove meaningless.

The Better Together campaign can certainly draw comfort from this poll, though they should avoid complacency. Such a dramatic shift in one year could be reversed by a different set of circumstances.

As for the SNP, its campaign is barely under way and, though it clearly has much work to do, time remains on its side.