TODAY's TNS-BMRB poll, published exclusively in The Herald, shows the rising temperature of the political debate in the last three months has had little effect on the numbers for and against independence.

Support for independence has stalled at 28%, the level reached in October, which was the lowest in the five years the SNP has been in power at Holyrood. At that time, backing for the Union reached a high of 53%, dropped to 48% in December and remains at that level this month, giving the pro-UK campaign a 20-point lead. The inclusion of those aged 16-17 in the poll for the first time appears to have had no impact, despite an expectation on the part of the Yes camp that younger people are more likely to support independence. That leaves the most notable shift in the political sands the increase in "Don't knows" from 19% in October to 24% in January.

This has coincided with a new focus on the practical consequences of independence rather than debating the principle. The complexity of the issues has become more apparent. Whether Scotland would automatically be entitled to membership of the EU, the practical consequences of a currency union with the rest of the UK (with the Bank of England as lender of last resort) and the effect on employment and the economy of removing all UK submarines from Faslane have all been hotly disputed and widely discussed, but would appear to have had a negligible effect on voting intentions.

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The stability in support for the SNP after it was forced to admit it had not sought specific legal advice on whether an independent Scotland would be accepted as a member of the EU following a confusing television interview suggests that its hard-core support is around 28%. This compares with 38% in favour of independence in August 2011 following the Holyrood election in which the SNP gained from demonstrating competence in its first term in government. Against 48% intending to vote No, the Yes campaign faces a considerable challenge. Twenty months away from the referendum, however, it would be foolhardy for either side to read too much into voting intentions.

With the signing of the Edinburgh Agreement in October, which put the 2014 referendum on independence on a legal footing, attention is increasingly focused on the consequences of independence. This week's rubber-stamping of the Agreement at Westminster, however, has been preceded by a refusal by Scotland Secretary Michael Moore to "pre-negotiate" any terms and conditions for an independent Scotland in advance of the referendum. While the SNP insists that negotiations are in everyone's best interests and Mr Moore that beginning to unpick the fabric of the UK would be a betrayal of duty, voters are caught between claim and counter-claim.

At this stage in the process, an increase in the proportion of people who are undecided is likely to indicate that they are waiting for further information in order to weigh up the pros and cons for themselves. As far as the politicians are concerned, it is evidence that the quarter of the electorate who have still to make up their minds want to be convinced of the consequences before casting their votes. The challenge facing both sides now is to win over those who have decided to vote with their heads rather than their hearts.