Party conferences are all very well, but they are not always closely related to life on this planet. That doesn't usually matter, because life in the political bubble is interesting enough. It has characters, mad, bad, dangerous and lovable; it has fights, feuds and reconciliations; and endless speculation about who is up or down. In Labour's case, though, we will be treated to a particularly otherworldly experience this week, in that the one bubble-world question is hardly going to be discussed – namely whether Gordon Brown will still be Prime Minister at the election. Journalists will still use the conference as an excuse to interview each other about it. That will only add another layer of unreality to the proceedings. Not since the Conservative conference of 2003, in which Iain Duncan Smith's speech was interrupted by 17 standing ovations, will a party put on such a strenuous show of unity in defiance of the bubble's own obsessions.

Let us skip over the bubble, therefore, to Friday, when a real political event occurs that does have implications for life on this planet – or at least this European corner of it. At the end of this week the Irish will vote in a referendum on the Lisbon Treaty, with opinion polls suggesting that, this time, they will vote yes.

It could, of course, be argued that European politics is even more detached from Earth-bound matters than the Westminster bubble, and there is some truth in that. The detail of the Lisbon Treaty is technical and confusing. This supposed "tidying-up exercise", as Jack Straw once called it, has spawned a bureaucratic Hydra of heads: a president of the council, a president of the commission and a foreign and security high representative thing. None of the changes in the treaty will make any difference to the shape of our bananas or which side of the road we drive on.

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And yet there is a larger and simpler truth, which is that Britain's place in Europe is a basic alignment of our politics. It does make a real-world difference whether the British Government is working with the grain of the rest of Europe or against it. Which is why the response of British leaders to the Irish vote is so important. In my interview with David Miliband today, the Foreign Secretary is scathing about David Cameron's refusal to spell out his response to a yes vote. So far, the Conservative leader has clung to an ambiguous form of words: that, if the Lisbon Treaty is ratified, he "would not let the matter rest there". On Friday, that slogan is likely to become unsustainable. At that point, Cameron faces a series of choices. What to say, and when to say it? To anyone observing from the outside, both are no-brainers. He has to say that a Conservative government would accept the Lisbon Treaty now that it is likely to be ratified before the election. And he should say it within minutes of the result of the Irish vote becoming known. Waiting until his conference speech the following Thursday is the sort of thing that Gordon Brown would try to do.

Yet it looks as if Cameron will not say what he ought to say, in which case when he says it becomes irrelevant. I understand that the Conservative leadership is still hoping that, even if the Irish say yes, the treaty will be delayed by a legal challenge in the Czech courts. Vaclav Klaus, the Czech President, has not put his blob of sealing wax on the parchment yet, even though it has been approved by his parliament. Klaus is a Eurosceptic, whose party is a member of the Tories' new group in the European Parliament. But there is an air of wishful thinking about all this. British Eurosceptics have wound each other up for months about how the Lisbon Treaty could be derailed, seizing with an alarming lack of proportion on rogue polls in Ireland and a ruling by the German constitutional court that was reversed, as expected, last week. My knowledge of the Czech legal system consists of a novel by Franz Kafka, but Foreign Office officials who are paid to know about such things say that the challenge is "not a problem".

Miliband is not wrong to say that "it looks like the Tebbits, etc" won't let Cameron say, "Of course we've got to live with Lisbon." The transformation of the parliamentary Conservative Party into an almost exclusively Eurosceptic body is one of the longest-lasting and most poisonous legacies of Margaret Thatcher. Cameron himself is a gut sceptic, though he has the political wit to include the pro-European Kenneth Clarke in his Shadow Cabinet. But does he have the political courage to tell the rest of his party what it is so unwilling to hear?

Not yet. Hence the discussion, as we report today, about other referendums that a Conservative government could hold. Cameron as prime minister could not hold a referendum on the Lisbon Treaty once it has been ratified, as the Europhobes want: to repudiate the treaty after it has come into effect would be to leave the EU. Hence the compromise plan for a popular vote on other issues that might come up in future negotiations. It is not going to satisfy the hardliners.

Cameron's cowardice on Europe is part of a pattern. Painting with the broad brush of feel-good rhetoric, he is a moderniser, replacing the image of the Tories as right-wing zealots. But on specifics he is often cautious and conciliatory of his own base. There are two things he could do that would make it easier for soft-core Blairites to complete the transition from New Labour to liberal Conservative. One would be to accept the Lisbon Treaty; the other would be to ditch the promise of an inheritance tax cut for estates of up to £1m, as Kenneth Clarke tried to do earlier this year. He succeeded only in pushing it back towards the end of a first parliament.

As he prepares his conference speech, Cameron has been rereading the speeches delivered by Thatcher in 1978 and Tony Blair in 1996 before they came to power. Thatcher's in particular has struck him for its lack of policy and strength of values. But policy matters in the real world, and on Friday he faces a critical test.

John Rentoul's blog is at independent.co.uk/jrentoul