Urging voters to give the premier "a bloody nose" at next week's election on Tuesday, Brian Sedgemore- a Labour lawmaker for 27 years who is now retiring from parliament -said he had joined the smaller opposition Liberal Democrats, who opposed the March 2003 Iraq conflict.
"I urge everyone from the centre and left of British politics to give Blair a bloody nose at the general election and vote for the Liberal Democrats," Sedgemore told reporters.
The defection was not entirely unexpected - Sedgemore was a long-time Labour rebel who had consistently criticised the Iraq war - but was nonethless deeply uncomfortable for Blair, just nine days before Britain votes.
Also turning up the heat, the main opposition Conservatives launched a fresh poster campaign calling Blair a liar as the gloves come off ahead of polling day on 5 May.
Michael Howard, leader of the Conservatives, also powered in with a targeted attack on the prime minister's character.
"If he's prepared to lie to take us to war, he's prepared to lie to win an election," declared a new poster emblazoned with a picture of Blair's face.
Blair's decision to invade Iraq
alienated left-wing Labour voters
Howard defended the hard-hitting campaign during a visit to Birmingham, in the West Midlands.
"I'm a very direct person. I say it as it is," he told reporters.
"Character is an issue at this election. It is about trust."
Blair attacks Tories
Desperate to win back control of the debate with barely a week left to go until the election, Blair rallied the Labour Party
faithful on Tuesday evening.
"I simply say to the people of this country, understand this Tory strategy: it is devious, it is underhand, it is back door," Blair told an audience of some 800-to-900 people in the northern city of Liverpool.
"I do not believe that the people of this country want to go back to that Tory government"
British Prime Minister Tony Blair
"But let us make sure it is not effective. Let us make sure it does not work," he said, speaking passionately and without any prompt notes.
"I do not believe that the people of this country want to go back to that Tory government, but we have got to make sure that that does not happen.
"We have got to get every single person out to vote."
The embattled prime minister said he would counter every insult hurled at him by talking about a successful policy issue promoted by Labour.
"Today I am talking about education, tomorrow I am going to talk about education, on Thursday I am going to talk about the strong economy, at the weekend I am going to talk about the NHS," Blair said.
Kennedy denied that a vote for
him was a vote for the Tories
Although Labour remains ahead in the opinion polls, party strategists fret that Blair's decision to back the US-led war has alienated many traditional left-wing supporters as was demonstrated with the defection of Sedgemore.
The prime minister has repeatedly urged Labour supporters to get out and vote on 5 May, warning that abstentions or protest votes for the Liberal Democrats could let the main opposition Conservative Party sneak into power.
Third term likely
The Liberal Democrats, however, dismissed such scaremongering.
"You want to vote for what you believe in," party leader Charles Kennedy told ITV television.
"If you feel we best represent it, go and vote for it, because the one thing you can be sure about is that we will still be believing it the day after the election, not just the day before."
Despite the continued pressures, Blair looks well set to win a third consecutive term in office, a record for Labour.
An opinion poll for the Independent newspaper put support for Labour at 40% against 30% for the Conservatives and 21% for the Liberal Democrats, a result that would give Labour a massive House of Commons majority.