A CONQUERING Roman general would have been happy with the homecoming adulation accorded to Nicola Sturgeon. Cheering crowds of SNP activists turned out when she returned to the campaign trail in Edinburgh the morning after the night of her television leaders' debate triumph.

In Thursday's showdown, the First Minister displayed all the qualities Scots recognise in her: she was assured, persuasive, quick witted and formidably well-informed. By turns, she was steely and charming. A UK audience less familiar with her was, to a very large extent, won over.

She suffered only one sticky moment - one that will have been lost on viewers south of Farage's Wall. It came when Ed Miliband tackled her on the SNP's central economic policy, full fiscal autonomy. "You need to explain what that would mean for the people of Scotland," the Labour leader said.

The branch office had obviously been on the phone. A few hours before the leaders went on air in Salford, Ms Sturgeon was enduring altogether more torrid time facing questions about full fiscal autonomy at Holyrood. She was asked whether her MPs would seek to amend Labour's proposed Home Rule Bill - the legislation that would deliver the Smith Commission powers if Mr Miliband becomes Prime Minister - to achieve full fiscal autonomy. Ms Sturgeon could not say they would. Her reticence was in marked contrast to a month ago, when she said, yes, her party would seek to make Scotland "fully autonomous," fiscally and economically, "if the SNP has got significant clout in the next Westminster Parliament." Scenting a climbdown, her opponents accused of lacking the guts to pursue her own policy.

Full fiscal autonomy is beginning to look like a millstone around the Nationalist neck. The Institute for Fiscal Studies has pointed to a £7.6billion hole in the public finances if the Scottish Government had to rely entirely on taxes raised in Scotland to meet its spending commitments this year. Jim McColl, the businessman and member of Ms Sturgeon's Council of Economic Advisers, this week spoke of a transition to full fiscal autonomy by 2020. John Swinney, the Finance Secretary, talked about the seven year delay in introducing the tax powers devolved under the 2009 Scotland Act.

But even if there were a substantial delay, the numbers do not look good. Another think tank, Fiscal Affairs Scotland, has also looked at Scotland's fiscal balance post-oil price slump and post-Budget. This year, 2015/16, they reckoned Scotland would need to plug an £8billion hole and going forward it would get worse, rising to £8.8billion in 2019/20.

Remember the old referendum question: how would you vote if you could be certain you would be £500 better or worse off under independence? In 2013/14, the latest year for which we have actual figures, rather than projections, you would have been £800 worse off. Fiscal Affairs Scotland's forecast for 2019/20 is £1600 worse off.

The gap would have to be plugged by borrowing, tax rises or spending cuts (or a combination of all three) unless a turbo-charged economy started filling Scottish treasury coffers with extra revenues. Mr Swinney and Mr McColl insist this would happen. No-one, neither the Scottish Government nor the think tanks, has tried to assess accurately the kind of growth required but it would be high. Implausibly high in the view of some economists. "Fantasy economics," say Ms Sturgeon's opponents. Unless she can marry full fiscal autonomy to a compelling growth plan and convincing transition timetable (or something truly dramatic happens to the price of oil in the next 12 months), Scotland's worsening finances will be a factor when it comes to deciding whether to include the promise of a second independence referendum in the SNP's Holyrood manifesto next year.

In the meantime, Labour and the Conservatives have identified full fiscal autonomy as the SNP's biggest weakness and are attacking it relentlessly. They are right to interrogate a key SNP demand but do voters fear a "Barnett bombshell," as the Labour posters say? Do they believe there is a pressing danger of "austerity max" under the SNP's plan? If we are heading for a situation where a minority government seeks support from smaller parties on an issue-by-issue basis, it is very hard to see an overall Commons majority for devo max for Scotland, assuming the SNP even decides to push for it. Labour insiders insist the message is starting to cut through on the doorsteps but perhaps a hypothetical Barnett bombshell fails to alarm people as much as they think. Its impact on this election, never mind the next, is unclear.