Part of the answer may be to contribute primarily in a range of areas - such as international geopolitics and the politics of climate change - in which he has expertise but which are not central areas of domestic political contention, and also to focus on the politics of organisation, and the 'movement for change' politics which both Milibands see as central to Labour's future success.
Economic strategy and the budget deficit, tax, and the future of public services, may prove trickier. But it would be a shame if that understandable instinct not to be seen to rock the boat meant having to shy away from engagement in issues of the broad aims and values and political direction of the party. It would be valuable to the party's debates about values and politics if he - along with all of the other major figures, whether on the frontbenches or (like David Miliband and Jon Cruddas) from the backbenches, do open up these debates during 2011, and find ways to do so which engage party and public audiences in the content rather than personalities.
Tonight's LSE/Political Quarterly lecture demonstrates a willingness to engage in debates about Labour's future existential political challenges, as shared across Europe by social democrats. It is well worth reading in full.
There is a clear Miliband to Miliband message in the crucial passage of the speech, though this could also be read as in part offering some Milibandite convergence even as articulating what Miliband the Elder sees as a crucial future strategic choice for the party.
So if we know who we have lost, and have some idea of why we have lost them, what next?
One conclusion is that only by reversing out of the Third Way cul de sac can the centre left find avenues for advance. It is certainly true that the centre-left governments of the 1990s were good at helping the poorest benefit from economic expansion, not good enough at figuring out how to spur that expansion. They were good at preaching responsibility for those on welfare, not good enough at demanding responsibility from those at the top of society. They were good at the analysis of an enabling state, but not good enough at bringing it about; good at the rhetoric of public sector reform, not good enough at delineating how both planning and markets are necessary for an effective public sector. And they were good at building electoral machines, not good enough at building movements of social change.
But my strategic view is essentially the opposite. The revisionism that was entailed in the renewal of the left parties in the 1990s was essential for them to become viable. It is not the new doctrines of the 1990s that made these parties unviable; it is that these doctrines staved off unviability, for parties that had become practised at losing elections in the 1970s and 1980s. The good things about progressive politics in the 1990s – a radicalism when it came to doctrine, new thinking about national and international reform, a finely tuned eye and ear for social and technological change, decisive engagement with people’s needs on difficult issues like crime and security, a readiness to pursue social justice in new ways, a strong sense of international responsibility, and a record that did leave the countries they governed fairer but also better prepared for the modern world - are the basis of winning again.
In other words, only a post New Labour brand of European social democracy, building on success, not a pre New Labour stance, can address the weaknesses that were left and exist today.
For David Miliband here goes perhaps somewhat further than he did in the leadership contest in articulating where he agrees with the critique of 'third way' social democracy of the 1990s. While rejecting the idea that the centre-left should "reverse out of the third way cul de sac", David Miliband does essentially voice and agree with several aspects of a critique which he probably associates with his brother, the new leader, about the limits of an excessively technocratic political project which did too little to build movements for change.
The second part of this passage makes the case for a further round of revisionist renewal of social democracy, rather than a retreat to the status quo ante. I don't think that issue about the direction of travel is a difference between the Milibands - indeed, to some extent, Ed Miliband's approach can be seen as being about recapturing the early breadth and confidence of mid-1990s New Labour from a later narrowing.
One of David Miliband's central themes in this speech, close in its advocacy of Tawney to several interventions by Jon Cruddas, is that values and political strategy will be more important in the review than policy.
And the speech does go on to engage with several important and difficult issues - including on the electoral base for social democracy, the role and limits of the state, and the balance between taxation and other means of social progress - which will be central to the policy review, and to the political choices for the party in opposition.
Mili-D's argument here has some common features with Peter Mandelson's argument in the new introduction to the paperback edition of his Third Man memoir that he does not think the risk of a leftwards lurch is a serious one, but that he is more concerned by a large section of the party becoming prone to 'one more heave' thinking.
In other words, the danger is less the return to a Kinnockite or pre-Kinnockite party. Rather, it is the post-1992 modernisers' critique of John Smith which is again relevant. But the political etymology of the 'one more heave' phrase perhaps demonstrates that this is not an open or shut argument. Though New Labour modernisers were right about how far the party had fallen short in 1992, they were too anxious in their war on complacency in 1997 (and even more so in being afraid of defeat in 2001). Had John Smith lived to 1997 and beyond, he would have won a thumping triple figure majority over John Major, if probably a slightly smaller one, and rather more in the style of Clement Attlee than Tony Blair.
A (weaker) case could be constructed too for "one more heave" now - it would very likely be enough to secure Labour a hung Parliament, quite possibly as the larger party, though certainly not a majority.
That "one more heave" is not the answer is also a point of Milibandite convergence.
This has informed Ed Miliband's argument as to why a post-Gordon Brown return to the eternal verities of New Labour will prove insuffient. (That is why “one more heave” just won’t do", as he said in the campaign, and again in his Fabian new year conference speech in January).
That is David Miliband's argument tonight too, that the difficulties of social democratic parties can not be put down to contingent factors of luck and leadership, and that post-New Labour politics can not hope to resurrect and reconstruct as it was a post-war social democracy which is in crisis in terms of electoral demography, economics and political organisation.
Neither they - nor anybody else - has yet done more than to sketch some early contours of an alternative. That an alternative is necessary should not be in doubt.