One weakness of Tony Blair's audacious but unfulfilled ambition to realign the British centre-left was the sense that 'the project' appealed in part because it could liberate him from the party platform to which he owed his premiership; a suspicion strengthened by his penchant for dressing to the right on the international stage.

Yet the 'progressive dilemma' to which Blair was responding was genuine. Since nobody doubts that Gordon Brown is of the Labour tribe, this leader may prove better placed to reach out beyond it.

The Prime Minister's challenge this Autumn is to set out the agenda which can turn the 'Brown bounce' into a realignment of British politics. Our Fabian Equality poll shows how political leadership could place a 'progressive consensus' within our grasp, by mobilising latent public support for a fairer Britain. But this will depend on Brown reinvigorating his party as a campaigning force, and ensuring that he puts the mission of extending opportunity and narrow inequalities at the heart of his bid for re-election, so that a fourth Labour victory would clearly define a new social democratic centre-ground in British politics.

Brown should also make further efforts to reach out, with a practical agenda to pursue progressive pluralism.

Firstly, the Governance of Britain consultation should be used to put electoral reform back on the agenda. Brown should indicate he would support changing to the Alternative Vote in a national referendum, as part of a democracy package in Labour's next manifesto including a second chamber which is 80 per cent elected by PR, and electoral reform for local government. Supporters of PR should find that a compromise worth making to build the historically elusive consensus for reform.

Secondly, the Prime Minister should engage all progressive opinion – from Labour members, unions and other affiliates to LibDems, Greens, non-party campaigners and progressive media – in a series of public debates on issues where a distinctive progressive approach to the national interest should cross party boundaries. The themes should be mutually agreed but natural contenders would include climate change; democratic reform; security and liberty; engagement with British Muslim communities; and Britain's place in Europe. Labour should ensure that ending child poverty and narrowing the class divide in education are central themes.

Finally, Brown should link this broader public engagement with Parliamentary scrutiny and deeper Westminster debates, inviting the LibDem frontbench to return to a joint Cabinet Committee to discuss any or all of these key progressive issues.

That would not be universally popular. Brown should challenge sceptics in the Labour party and trade unions to face out, and build alliances which can win the public argument for a more equal Britain.

It is not clear either whether or not Britain's second largest centre-left party would have any appetite for constructive engagement. The Liberal Democrats are often charged with being a 'franchise party', running on very different platforms around Britain. But, nationally, their effective championing of key progressive causes commands respect: raising once unfashionable issues of constitutional reform and the environment, and articulating minority positions on civil liberties and crime.

However, their ambition to be more than a pressure group is unproven. Brown's own offer of ministerial posts to individual LibDem peers, without any formal party arrangement, proved too mould-breaking of British political convention for the third party. Yet the LibDems rejected all of their coalition options following multi-party PR elections in Wales and Scotland too. When even Ian Paisley and Martin McGuiness seek to make devolution work in Northern Ireland, these serial LibDem refusals offer a poor advertisement for the grown-up politics of compromise. Their leader even seems to have elevated standing aside into a new 'Ming doctrine' declaring that 'we will not trade principled opposition for ministerial briefcases'.

Before their on-off 'nineties romance fell away, Paddy Ashdown famously asked Tony Blair, 'Are you a control freak or a pluralist? Gordon Brown should go the extra mile in to prove genuinely pluralist intent, engaging substantively with progressive opinion of all parties and none. That would also offer the LibDems a potentially historic test: will they choose progressive pluralism or purist impotence? SK