The leftwing backbencher John McDonnell will today launch his campaign for the Labour leadership.
The MP for Harlington and Hayes - who has described Labour showdowns over Tony Blair's departure date as being like an episode of The Sopranos - is the only officially declared candidate for the forthcoming leadership election so far.
Following mounting pressure and yesterday's wave of resignations from his government, Mr Blair is expected to set a timetable for his departure today.
Mr McDonnell, who is keen to undo some of the prime minister's public service reforms, will be backed by Tony Benn, the veteran Labour party member and former cabinet minister who failed in a 1988 challenge to Neil Kinnock.
Other speakers endorsing his candidature in Manchester will include the retired MP Alice Mahon and Jeremy Dear, the general secretary of the National Union of Journalists.
No sitting MPs are due to appear at the event, which has been organised by the local branch of the Transport and General Workers Union.
Nevertheless, Mr McDonnell, who chairs the leftwing Campaign Group of Labour MPs, is confident he can gather the 44 signatures necessary to back his leadership bid.
He said he was keen to "open the debate" on Labour policies, and believes he is the right man to bring disaffected party members back to the fold and build a "broad coalition" of support.
Mr McDonnell, who orchestrated a trade union campaign opposing the privatisation of public services earlier this year, is due to speak at a number of fringe events at next week's TUC conference in Brighton to galvanise support from unions.
Affiliated union members will have an individual say in the leadership challenge. Mr Blair and Gordon Brown are also expected to court the unions early in the week.
"Now that a leadership election is inevitable over the next six to nine months, if not sooner, I am launching a process by which rank and file Labour party members and trade unionists can have their say on the fundamental debate around the future of the Labour party," Mr McDonnell said.
"I am insisting that this debate will not be confined to the infighting among the London new Labour elite."
He said tonight's meeting would "launch a national campaign for a radical break with the failed policies of new Labour", adding that changing leaders would "not be enough to save Labour at the next election".
Mr McDonnell said Manchester had been chosen as the starting point for his tour because the city is important to Labour voters. He said it had played a "significant role in the history of the Labour party as one of our heartlands, and has stuck with us through thick and thin."