The British Prime Minister, Mr Blair, and the Conservative leader, Mr William Hague, are locked in battle for the heart of Britain after the clearest signal yet that the Labour government intends to join the euro.
Despite his initial reluctance to see Europe as a major election issue, the Prime Minister yesterday charged the campaign with a bullish assertion that he could win a referendum on British membership of the European single currency.
The Conservative leader conceded that Mr Blair well might, repeating his charge that a second-term Blair government would "rig" the promised referendum, and insisting that June 7th is in real terms decision day for Britain.
A Labour victory next month, declared Mr Hague, would "sink" the pound.
Having picked up Mr Hague's euro gauntlet, Mr Blair then devoted his most important speech of the campaign so far to the "patriotic" case for British engagement at the heart of Europe.
Speaking in Edinburgh, he said patriotism should be reflected in recognition of the interdependence of the modern world rather than a retreat into isolationism.
"The case I make is simple. To stand up for Britain in the modern world means to be an active, engaged partner in the alliances of which we are a member. Isolation and marginalism in Europe are not patriotic, but the denial of our true national interest," he said.
Earlier Mr Blair made it clear that, while the Treasury was "guardian" of the five economic tests to be met before the government committed to a referendum, this did not amount to a veto in the hands of the Chancellor, Mr Gordon Brown.
And despite last weekend's evidence from a former aide to Mr Peter Mandelson about a secret pact which would see Mr Blair make way for Mr Brown toward the end of a second term, Mr Blair told the Financial Times he intended to serve as Prime Minister for the duration of the next parliament.
As the UK Independence Party opened its national campaign for British withdrawal from the EU, Mr Blair dismissed polling evidence of mounting scepticism about the whole European project. "When you probe deeply, most people in this country believe it would be very foolish indeed for us to exit from the EU," he said.
Asserting the need for far more confidence in making the case for a strong Britain in a strong Europe, the Prime Minister signalled his intention to be "very active" in shaping that debate after the general election.
Apparent hostility toward the euro, he suggested, reflected the fact that the government itself was not at present suggesting membership. Once that situation changed, he said, so too would the debate.
Asked if he therefore believed a referendum could be won, Mr Blair replied: "Of course, provided you mount the argument well, provided we are setting out why it is economically and politically in Britain's interests."
Nor was the government ignoring the constitutional implications of giving up its currency. "I have often been misunderstood about this. I do not say there are no political and constitutional arguments. I simply say that as a government we have resolved those in favour of being in the single currency," Mr Blair said.
The Liberal Democrats leader, Mr Charles Kennedy, said he hoped Mr Blair's statement was significant, while warning that the Prime Minister and Chancellor had been "playing hard cop, soft cop" on the euro for four years.
Mr Hague said he had no doubt that the Prime Minister's words were significant. "I think it's clearer than ever that he wants to ditch the pound," he told reporters. "If Labour win the election the pound is doomed."
The Tory leader rejected Mr Blair's suggestion that it would be "patriotic" to join the euro if it was in the country's economic interests. "It is a funny sort of patriotism to say we can't run our own affairs," he insisted.
"We can keep our own currency, and I think the majority of the people in this country wish to do so. I wish to do so and I hope the people will make that clear in this general election."
Despite currently large majorities declared against, Mr Hague conceded it was possible Britain would vote Yes in a referendum following a Labour victory.
Labour, he said, would use the power of the government machine, and its ability to determine the question, timing and funding for a referendum campaign, to engineer a "temporary majority" in favour of British membership of the single currency.
"By hook or by crook he [Mr Blair] would try to get us into the euro. That is why this election is the key vote on keeping the pound," Mr Hague said.