Prime Minister Tony Blair today launched a nationwide, cross-party campaign to sing the virtues of Britain's membership in the European Union--just as the leading opposition party turns sharply toward the anti-Europe camp.

Presiding over a lavish rally for business and union leaders, Blair was flanked on the stage by fellow members of his ruling Labor Party, by two senior grandees of the opposition Conservative Party and by the leader of the Liberal Democrats, Britain's third major political party. The members of this "patriotic alliance," Blair said, "know that by being part of Europe we advance our own self-interest as a British nation."

Blair's new campaign sets the stage for the biggest debate over Britain's place in the European Union since the United Kingdom joined the community in 1973. It comes at a time when the Conservative Party leadership has adopted a political strategy centered on consistent Europe-bashing.

Still, Blair is clearly not ready to argue that Britain should adopt the new single currency, the euro, which became the legal tender of 11 of the EU's 15 member states on Jan. 1. He barely mentioned the euro today, simply repeating his standard refrain that Britain will consider joining the monetary union "when and if economic conditions are right."

As he so often does, the prime minister seems to have located himself precisely in harmony with public opinion. Surveys here--including a poll released today--suggest that a big majority of Britons want to remain in the European Union.

But only about a fifth of the population is ready now to drop the pound and adopt the euro instead, the polls indicate.

As an island country that has closer historical and cultural ties to the United States than do the continental European nations, Britain has always had a tentative relationship with the EU.

The advent of the euro has heightened the sense of national indecision over whether Britain can stand on its own with its own currency.

For most of the 1990s, the two major parties were pretty much in sync on the euro, and both backed British membership in the EU. Labor's position on the single currency has been described as "wait and see," with Blair promising no decision until around 2002. The Conservative position has been "wait a little longer and see," with a decision delayed until around 2008.

But this fall the two parties have moved in opposite directions.

Leading members of Blair's cabinet have suggested that Britain should and will adopt the euro sooner rather than later. Meanwhile, influential voices in the Conservative Party have been taking a tough new line that is not only anti-euro but also anti-Europe.

At the party's national convention last week, the Tories announced policies that suggest the possibility of outright withdrawal from the EU.

The tough new anti-Europe rhetoric at the convention prompted an outpouring of criticism from several moderate Conservatives. The moderates, in turn, were savaged by the more extreme members of the party. Once again the Conservatives were engaged in an angry intramural donnybrook--the very conduct that was largely responsible for their huge defeat in the 1997 general election.

By convincing two of the moderate Tory elders--Kenneth Clarke and Michael Heseltine--to join him on the platform today at his "Britain in Europe" rally, Blair highlighted the Conservatives' divisions.

And he set up an ongoing debate on a question that is never as obvious here as the geography books suggest: Is Britain part of Europe?