Party chiefs are struggling to prepare alternative policy blueprints to unveil to grassroots activists, as the mood in Ireland suggests the vote on October 2 could go either way.
If the Irish vote No, it will give a huge boost to David Cameron's campaign to derail the EU Treaty and will open the way for the Tory leader to promise the British people a referendum on the issue.
I really don't buy this from the way that Cameron talks. I sense that he's actually hoping that Ireland vote for it in order to avoid having a referendum which would doubtless be voted against and create a whole load of Tory divisions.
However, if Ireland votes Yes, the Treaty will move a step closer to becoming law before the end of the year, and the Tories will be plunged into a deep dilemma as to how they can possibly overturn it.
Both Mr Cameron and William Hague, the shadow foreign secretary, have already said that they would "not let matters rest" if the Treaty was ratified by all EU member states before a Tory government came to power.
That's precisely why I'm suspicious of Cameron over this. Because a statement about "not letting matters rest" is a pretty empty promise. There's enough lawyers and Constitutional experts in the Conservative Party who can advise Cameron of his possible actions that they could right know what they could do. "Not Letting Matters Rest" sounds like spin to let it look like they care about it while not actually showing any teeth.
And no, the promises of something being in a manifesto aren't enough. Brown isn't going to steal this, so just come out with it and be done.
A poll for the Irish Times last week, the first since the campaign was launched, found that the Yes campaign was still in the lead, with 46 per cent support, but had dropped eight points since the previous poll in May.
The No campaign had 29 per cent support, up by one point over the summer, whilst the size of the Don't Know category had increased sharply by seven points to 25 per cent.
Pat Cox, campaign director of Ireland for Europe, a group promoting ratification, appeared nervous, saying: "Ireland is a very different place today to what it was a year ago. The financial crisis has rocked our confidence. We are reeling from a series of body blows over the last 12 months.
"There are those on the No side who will seek to exploit our present uncertainty to encourage the Irish people to vote against our own interests and reject the Treaty."
It doesn't sound good, but it still sounds possible. A loss of some of that lead and it might just tip a no.
Then Cameron's really in the shit.