Politics can be a punishing, unforgiving business but occasionally it permits redemption too. At historical moments which define a nation, political leaders can sometimes rise above the constraints their own character traits, personal histories and partisan allegiances impose on them.

The rehabilitation of Gordon Brown's reputation advanced a step further today as he gave a vintage, at times volcanic speech to a No rally in Glasgow. On Friday, the same performative test that Brown has just passed will be faced by David Cameron and Ed Miliband. They will be called on to speak in terms appropriate to the gravity of the decision the Scottish people will have taken. Few if any us ever get to cast a vote that simultaneously embodies and expresses our popular sovereignty; we ordinarily exist in humdrum, linear historical time, not at foundational moments in which a political community is born. Tomorrow, whether they vote to stay or go, the Scottish people will mark their ballot papers not just as an electorate but as a sovereign body.

If it is a Yes vote for independence, our political leaders will have to find a register in which to express a sombre acknowledgment of fate, an obligation of fidelity to the choice Scotland has made, and a seriousness of purpose by which to a negotiate a painful separation. They will have to acknowledge the loss that many of their compatriots feel, while at the same time recognising that for millions of Scots, a vote for independence will be a cause for celebration. Their personal fortunes and sectional interests will weigh on their minds, but must not hang in their words.

If it is a No vote, they will face the equally daunting task of charting a future direction for the union. To start, they will need to celebrate and harness the extraordinary energy and democratic vibrancy of the referendum campaign. Turnout will be very high, and it will be the culmination of a deeper and more extensive process of democratic deliberation than we have seen in the UK for decades. That energy should be channelled into a process of democratic renewal for the union as a whole, and for England in particular. The calls for a citizen assembly or equivalent processes in England must be heeded, enabling citizens, civic organisations and political representatives to bring forward their ambitions for reform to English governance, while a subsequent UK-wide constitutional convention or Council of the Union should deliberate on reforms that are needed to Westminster institutions and relations between the nations. Ireland and Iceland offer precedents here, but there are a myriad of specific ways these processes could be configured. The key issue is that party interests must be subordinated to a general agreement that this is the terrain onto which our politics must move – even if there are fierce political battles over specific policy choices in the months ahead.