Consider some of the possible scenarios. If there is a "No" vote, the authority of the Labour government would be greatly and, possibly fatally, undermined. Almost certainly, Tony Blair would resign soon afterwards, but those left behind would be associated with the defeat as well. There would be a collective and morale-sapping sense of rejection. Contrary to popular perceptions, this is not an arrogant and out of touch government. Even after two terms, ministers feel like impostors who have disturbed temporarily the natural order in Britain where the Conservatives rule with only brief interludes. A no vote would reinforce that sense of doomed insecurity.
Assuming Gordon Brown is in the Cabinet, there is no way he could, or would, choose to keep a low profile during such a campaign. A no vote would be as big a blow to him as it would be for Mr Blair. His authority would suffer at the very moment he acquired the crown.
Such an outcome would be a big boost to the Eurosceptics in the Conservative Party. A Conservative leader would have been on the winning side, a sensation so rare in recent times that Eurosceptics would feel almost giddily vindicated. The pro-Europeans in the party, already keeping their heads down, will lose any remaining political ammunition. The only debate within the party would be over whether it was Eurosceptic enough.
A yes vote would be almost as dramatic. Mr Blair would be in a position to make a triumphant exit, having secured a victory on Europe. We should not get carried away. I am told authoritatively that Mr Blair has given up hope of achieving what he once described as his historic objective: ending Britain's ambiguous relationship with Europe. Mr Blair has concluded a yes vote will not end the ambiguity. In his view, that will happen only when the media changes the uniquely poisonous way it reports matters relating to Europe. He sees no sign of that happening in the near future. Even so, a yes vote would be a rare boost for the pro-European cause and would force the Conservatives to reconsider their Euro-scepticism. Currently, Britain is the only country in the European Union where the main opposition party is virulently Eurosceptic.
There are many other questions whirling around the outcome of the referendum. To take two possibly highly charged outcomes: What if Scotland votes yes, but the outcome is no? What if the overall result is yes, but England votes no?
When Europe stirs, it is always the equivalent of a great monster wreaking havoc on neurotic British politicians. Having won four elections in a row, the Conservatives never recovered from their divisions over Maastricht and the departure from the Exchange Rate Mechanism. The wounds over the traumatic exit from the ERM are still so raw that Treasury papers relating to the event are the first source of controversy caused by the new Freedom of Information Act. Before that, Labour was doomed to 18 years of opposition, partly because of a schism over Europe. It is possible that a no vote in another EU country will scupper the whole project, but Mr Blair expects that the other countries will vote yes, and the referendum in Britain will be held. Europe is back on the British political scene and something big is about to happen again.
The subtle challenge for pro-Europeans is to convey the high stakes while sending out the seemingly contradictory message that the revised constitution does not change a great deal. Yesterday in the Commons, the Foreign Secretary, Jack Straw, showed how it was possible to adopt this unavoidably convoluted position. Mr Straw was opening a debate on the Bill that will give the go-ahead for the referendum. Yes, Mr Straw declared earnestly, he would prefer the vote to be about the treaty and the treaty alone because it is a good treaty.
Note that he referred to the controversial document as a treaty rather than a constitution. It is a treaty, like the others that were passed without a referendum.
Even so, Mr Straw went on to note regretfully, there are Conservatives planning to use the campaign as a way of bringing about withdrawal from the EU. It is they who are raising the stakes. Mr Straw described the choice: "Strength in our kind of Europe or a step into the unknown". He seeks to stir up a calm panic.
In doing so, he is not only following an astute strategy but also stating the truth, which always helps. If there is a no vote, anything could happen. Britain could be kicked out of the EU, choose to leave the EU, formalise a new distant relationship with the EU, or it could stagger guiltily around a big unpredictable muddle throughout Europe. In contrast, a yes vote does not change very much. The choice becomes similar to the referendum in 1975 when the yes vote endorsed Britain's "continuing" membership of the European Union. The no vote then was the bigger risk.
The other weapon for the yes campaign now is the internal division in the no campaign. The Conservatives' policy, one that will not withstand the intense scrutiny of a referendum campaign, is to remain in the EU, but to renegotiate Britain's membership although no other EU member supports them. Other parts of the no campaign want to pull out, a position shared by some Conservative MPs. Currently, the yes campaign lacks the exuberant, hyperactive confidence of its opponents and there are considerable tensions within the Government over the best strategy, but at least it is united in the outcome it seeks in terms of Britain's relationship with the rest of Europe.
Even so, the yes campaign will need to deploy its most powerful forces to counter the newspapers that will lead the no campaign. This is where Mr Blair's post-election cabinet reshuffle acquires more than usual significance. Recently, it has been reported that Gordon Brown is warming to the idea of a move to the Foreign Office. Even if this were the case, which I doubt, Mr Brown would not like the brief and knows he would not like the brief. If he accepted the demotion, he would be an unenthusiastic, and out of sorts, foreign secretary. Mr Brown would be at his most convincing in a referendum campaign as a successful chancellor known to have doubts about elements of the European project. Similarly, Mr Straw is not known as a Euro enthusiast. His wholehearted advocacy as a foreign secretary would be another potent weapon.
If Mr Blair wants to win the referendum, he would be well advised to keep Mr Brown and Mr Straw in their current posts. The referendum is winnable, but only if everyone plays their parts and is at ease with the parts they are asked to play.