Every now and then an unexpected souring in the public mood can quite suddenly overturn all the conventional assumptions about a nation's politics. We may be on the cusp of just such a seismic shift. I am not referring, though, to the outcome of the general election. If the polls are to be believed, the important vote will take place a few weeks later on the other side of the Channel. France's referendum on the European Union's constitutional treaty promises to transform not just French but British politics.

Europe has scarcely figured in Britain's election campaign. I did catch Michael Howard on Monday telling industrialists that the Conservatives would never join the euro and would declare all-out war on the bureaucrats of Brussels. Speaking from the same podium, Gordon Brown, the chancellor, seemed to be saying something remarkably similar. Look closely, meanwhile, at the finest print in the Liberal Democrats' policy platform and you will find only the merest hint of its federalist inclinations. That is about the sum of it.

Its absence as a campaign issue belies Europe's capacity to shape British politics. Margaret Thatcher's fall, after all, was triggered by her No, No, No to the integrationist ambitions of Jacques Delors' European Commission. John Major saw the Conservative party break in two over the Maastricht treaty. Go back a bit further and the argument over membership of the then the common market was a fair proxy for the division between left and right in the old Labour party. Fast forward to the 2001 election and the hapless William Hague built the Tory campaign on a countdown to save the pound from the clutches of the single currency.

Even a month or two ago, the received wisdom was that the present silence on the subject of Britain's relationship with its neighbours was merely a lull before the storm. If Mr Howard defied the polls and won on May 5, the country would be thrown into confrontation with the rest of the EU as the new prime minister rejected the constitutional treaty and demanded new opt-outs from EU policies. If, as now seems even more likely, Mr Blair was returned to 10 Downing Street, Europe would soon define the final phase of his premiership. Mr Blair would almost immediately face an uphill struggle to win a British referendum on the treaty scheduled for next summer. Win, or more likely lose, that vote would probably serve as the curtain call on his premiership.

A month, though, is a lifetime in politics. The French No to the treaty signalled by the opinion polls would upend everything. Some European leaders are already insisting that the ratification process would have to proceed in other EU states. In reality, French rejection would kill the treaty. As far as I can tell, no one in Paris believes that Jacques Chirac would be willing to present it to the electorate again before the next presidential election in two years' time. How then could other governments proceed to ratify a treaty that the French president had declared to be dead? This is not a British problem. Forget Mr Blair here, and ask if Poland, the Czech Republic, Ireland and Denmark could secure the backing of their voters for the treaty.

So the French vote No and, in one bound, Mr Blair is free? Well, yes and no. Yes, because French rejection would lift the threat of a referendum defeat for Mr Blair and of Britain being pushed to the margins of the EU. It would thus remove the issue from calculations about Mr Blair's departure. No, because the choices faced by Britain as to its role in Europe would not simply go away.

Mr Chirac would blame, curiously enough, Mr Blair. It was the British prime minister's surprise decision last year to call a referendum that forced the French president to do likewise. This, according to the French version of events, after Mr Blair had privately pleaded with Mr Chirac over many months to rejects calls in France for a plebiscite. To make matters worse, the prime minister did not even bother to tell the president when he changed his mind and announced the British vote. Little wonder Anglo-French relations have since been less than cordiale.

As it happens, Mr Blair takes over as president, and thus conciliator-in-chief, of the EU's council of ministers in July. A French No would threaten to reopen the fissures exposed by the Iraq war. He might be tempted to put himself again at the head of "new Europe", allowing "old Europe" to regroup around the Franco-German alliance. The more intelligent course would be to seek to hold the 25 together, probably on the basis of a slimmed down version of the treaty. Neither would be comfortable. Both would bring Europe back into British politics.

There is, of course, another possibility. The French people could change their minds again and decide there are better ways to punish Mr Chirac. By the same token, it is possible that the British electorate will put Mr Howard into Downing Street on May 5. It just does not look like that from here.

philip.stephens@ft.com

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