In his conclusion to Parliamentary Socialism, Ralph Miliband, father of brothers David and Ed, posed a challenge that is more pertinent than ever. He argued that the alternative to becoming a genuinely socialist party would be 'the kind of slow but sure decline which - deservedly - affects parties that have ceased to serve any distinctive purpose'.
'Slow but sure decline' has certainly been the trend under New Labour, with membership falling to an all-time low. However, in the week following the election, 12,000 people joined the party, as if they sensed the danger faced by the left at the end of the New Labour era.
Over the following weekend, a slew of post-election conferences from the Fabian Society to the Labour Representation Committee discussed Labour's future with an atmosphere of optimism - bordering on outright relief - that might seem curious for a party that has just lost 90 seats and been ousted from power.
Old hands and young bloods alike are keen to break from the New Labour tradition. Former cabinet minister Jon Trickett emphasised that a revitalised movement must 'openly say that Iraq was a mistake, and never will such a thing happen again', advising party members to admit that Labour also 'operated with completely the wrong economic paradigm'. Labour's new MPs have arrived at Westminster with deeply critical perspectives on Blair and Brown's positions on Iraq, privatisation and immigration. At the Progressive London conference, Lisa Nandy, newly elected to represent Wigan, described her relief at being able speak freely on such matters.
Nandy's successful campaign was part of a new strand of Labour thinking that 'keeps in touch with left-wing people who may not be Labour born and bred'. Many Wigan voters were 'motivated by the memory of what the Tories did to the north west in the eighties and nineties, and came to work with us. They were also motivated by equality issues and the Tories' homophobia,' said Nandy. While David and Ed Miliband's campaign speeches have sounded rather like bids to be chief executive of an ailing company, there is also a broader, more plural view of a labour movement opening up among the young and at the grassroots - a view that doesn't require a party card and that is based on joint action.
'After the election I had a socialist awakening, and I realised that the Liberals don't have the type of grass-roots socialist politics that interest me - so I joined the Labour party,' said Adam, 19, at the Fabian conference. 'But the party needs a leader who can unite the left without being tainted by the New Labour brand. I'm hoping that Ed Miliband can offer that kind of leadership, but I'm not sure.'
Billy Hayes, general secretary of the Communication Workers Union, stresses the need for the left to focus on the practical issues facing working people. 'Building resistance to the cuts has to be the priority,' he says. 'People don't realise how hard they are going to hit. The leadership will come from those at the front line, but I expect the Labour Party, along with the new leader, to offer support.'
The veteran Labour left-winger and leadership candidate John McDonnell agrees that Labour needs to relearn its ambition to improve the lives of ordinary people. 'The last general election seemed to be fought in a fantasy world,' he says. 'None of the main three parties have any clear analysis of globalisation, and all have been co-conspirators in creating a global economic crisis and then expecting working people to pay for that crisis. There are alternatives to this, and the alternatives strike at the heart of the system itself - so it means systemic change, on an economic level, on a parliamentary level and on a party level, and a return to socialist awareness.'
After tramping around the post-election conferences, we went to Westminster to the Take Back Parliament rally for fair votes. Swathes of protesters decked in purple ribbons marched on Downing Street to turn in their petition for proportional referendum. Their desire is for a political system that is less centrist, less timid, and fairer in principle and practice to the unheard millions who have waited too long for politicians to take a stand. No mainstream party in its present state offers the radical change that this constitutional reform movement wishes to see - not even the Liberal Democrats.
If Labour could make the links between economic democracy, political democracy, workers' rights and civil liberties, then it might regain the distinct purpose that Miliband senior insisted was a condition of reversing its decline.
Hilary Wainwright is a member of Red Pepper's editorial collective and a fellow of the Transnational Institute.
Laurie Penny is a freelance journalist who blogs regularly for the New Statesman.