All three unionist parties have made or renewed promises that in the event of a No vote the Scottish Parliament would be given more powers.
Meanwhile, Labour has been making a concerted effort to convince us that the Union offers the brighter hope for a fairer, more equal society.
Unfortunately for Alistair Darling and his team, none of these messages is getting through. Since our last poll four weeks ago, the proportion who think independence would be good for Scotland’s economy has increased by three points to 38 per cent. The proportion who think it would be bad is down three points to 43 per cent.
Pessimists now outnumber optimists by just five points. Last September the figure was 17 points.
The warnings from and about business are evidently either being ignored or disbelieved.
Voters are becoming increasingly convinced that an independent Scotland would be a more equal country. No less than 36 per cent are now of that view, up three points on February and no less than nine as compared with last September. Even those who voted Labour in 2011 are slightly more likely to think there would be less (27 per cent) than more inequality (22 per cent).
Meanwhile, although a quarter of voters fear pensions would be lower as a result of independence, the figure is no higher now than it was seven months ago.
Labour’s vision of the UK as a ‘sharing Union’ is evidently not one that comes readily to most voters’ minds.
Promises of more devolution have been thick in the air in the last fortnight. The former Labour Prime Minister, Gordon Brown, made a rare major speech on the subject, his Conservative successor, David Cameron, renewed his backing for the idea, while the final report of Labour’s Devolution Commission was published last Tuesday.
No less than 68 per cent of Scots agree that, in the event of a No vote, the Scottish Parliament should become primarily responsible for taxation and welfare. But just 39 per cent think Holyrood will actually be given more powers and responsibilities should Scotland vote No.
Even among those who want more powers, only just over two in five (42 per cent) believe the promises of more powers will be delivered. It is here perhaps that the unionists’ credibility gap is at its most striking.
The one piece of good news for the No side is that the vast and indeed increasing majority of No voters (81 per cent) are resolved to vote No even if by September they were to be convinced more devolution was not going to happen.
Even so, 10 per cent say they would switch to the Yes side, while another 10 per cent are not sure what they would do. Small though they might seem, these numbers could still be enough to tip the balance.
If all of the first group were to switch sides, together with half of the second, then bearing in mind, too, the inclination of more undecided voters to back Yes than to vote No, that would be sufficient to turn the referendum race into a virtual dead heat.
The lessons for the No side are clear. Frightening voters with messages of economic doom and gloom is not working.
• John Curtice is professor of politics at Strathclyde University