David Cameron's struggling to persuade Scottish voters that there's no going back from independence. It could be the No campaign's biggest failure.
They had it all worked out. For so many months, No 10 was able to deal with its numerous other crises safe in the knowledge that the Scottish independence headache was being kept safely in a neat little box. The referendum was a risk, yes, but a manageable one. The polls were reassuring. And they could take solace from the high-stakes nature of the contest: by going all-in and making this an irreversible decision, the British establishment could take comfort from its strategically sound decision of calling Scotland's bluff. There was no chance, it was felt, that the Scottish would vote 'Yes' in the vague hope of achieving some sort of devo-max.
Somehow, in the critical final stages of the campaign, that has changed. Two factors have suddenly transformed the race, turning dull complacency into urgent, grim panic.
The first is the polls. Sunday's four sets of numbers show the race remains too close to call. Panelbase has 'Yes' on 49 and 'No' on 51. Survation's phone poll giving 'No' an eight-point lead of 46-54 is balanced out by ICM Research's survey switching the numbers round, with 'Yes' eight points ahead on 54-46. Then there's Opinium, which has 'Yes' on 47 and 'No' on 53. The blind panic of last week may have subsided somewhat, but both results remain plausible with less than 100 hours to go until polling stations open.
The second factor is that of Scottish attitudes to what happens in the event of a 'Yes' vote. There is a sense that some Scots prepared to side with the nationalists might do so in spite of the fact they believe the union would not be dissolved, or even just to cause trouble.
A poll for the Sun on Sunday reveals the extent of this unfounded optimism. There's a genuine belief that it is the politicians of Westminster who are bluffing. Independence might be reversible if it doesn't work out, they think. The pound could remain Scotland's currency, in spite of the three British party leaders ruling out such an option. And Scottish people could continue to live and work in England, despite the English threatening to stick up border controls.
Together, these ideas are the biggest risk to unionist campaigners. Their existence is a terrible indictment of the Better Together campaign, which has pursued a relentlessly negative campaign while failing to address the real danger. So Cameron, who is in Scotland again on Monday for the tenth time this year, will have to work hard to get the message across.
"If Scotland votes yes, the UK will split, and we will go our separate ways forever," he's expected to say.
"We must be very clear there's no going back from this. No re-run. This is a once-and-for-all decision."
FOREVER. The message is clear enough. Actually, it's too clear. Scotland could very easily rejoin the UK in a century or so. It's why even this last-gasp attempt to underline the perils of a 'yes' vote is mistakenly ramping up the hyperbole. As the Sun on Sunday poll shows, there's a genuine scepticism that the dire warnings of the establishment wouldn't become reality.
If Scotland votes 'yes', a lengthy negotiation process takes place. There are so many question-marks over what would actually happen that the phrase 'constitutional crisis' would be understating it. Would the Scottish be entitled to vote in next year's general election - and could that cost Ed Miliband an overall majority? How much of Britain's assets and national debt would Scotland take on? When would independence actually take place?
So much of this is just a guessing game. But there is one precedent we can turn to in order to find out what Cameron does when handed an unexpected, catastrophic setback. Rewind 12 months ago, to that shocking Commons defeat over a military intervention in Syria, and you see how the government responds when presented with a disastrous reversal.
It starts with the response. Cameron, who sat in his place in the Commonswaiting for MPs to finish voting, was laughing and joking with colleagues before being handed a small slip of paper. He studied it quietly and carefully. After the result was read out, he stood up and read out what was written down: that the government had got the message loud and clear.
From seconds after the result, the machinery supporting Cameron had got underway, controlling his response and the enormous institutional sulk which it triggered. It got so bad that a year later - when backbenchers were genuinely in the mood for military action against the Islamic State - the government wasn't even thinking about trying again. It had conceded, adjusted, moved on, embraced the new normal. The government, hurt and embarrassed, had clammed up tighter than Gordon Brown's fist. It found its new position quickly and dug in fast. The same will happen later this week if more people vote 'Yes' than 'No'.
Right now, Scotland's marriage with the rest of the UK is on the rocks. We're at the 'please stay with us' pleading stage right now. All that changes on Thursday, when a decision will finally be taken one way or the other. If it's in favour of independence, those expressions of hopeful reconciliation will harden to ones of shocked determination. The divorce lawyers will be deployed - and the British government has its fair share of lawyers used to getting their way. If it comes to it, it's going to be bitter. It's going to be messy.
Today's Sun on Sunday poll reveals the gap into which this sorry episode could fall. Where Scottish voters expect 'Yes' entails flexibility, the English anticipate it will involve immigration controls, and a Scottish currency of its own, and a clean break for good.
Monday's speech will see Cameron doing his best to try and close this gulf in expectations, but it may be too little too late.
If the Scottish vote 'Yes' and their expectations of what that means are disappointed, all those involved could end up as losers.