Clegg places the LibDems firmly on the centre-left of the progressive spectrum, acknowledging that they share this space with Labour, and arguing that the 'progressive conservatism' will be revealed to be a contradiction in terms. However, this sets up an argument not for cooperation between progressive forces, but for the LibDems supplanting Labour as the dominant progressive party, reversing Labour's eclipse of the Liberal Party in the inter-war years in the last century.
James Graham has an interesting reading of the political strategy behind the pamphlet, arguing that it re-reverses the LibDem policy of "equidistance" between the major parties, which was ditched by Paddy Ashdown then readopted when the Blair-Ashdown 'project' of realignment by mutual cooperation fell through. I suspect the LibDem leadership would not quite agree with Grahams claim the pamphlet "has ruled out any chance of doing a deal with the Conservatives in the event of a hung parliament", but the intention will have been to make the prospect more distant. Tonally, the barriers to working with Labour are stressed more.
Clegg's reading of political history is an interesting one, though I think it is rather debatable.
On the ideological underpinnings of Labour and the LibDems, he is more interesting on his own party than in his description of Labour. Though this is asking a lot (and almost certainly too much of a party leader), the success of this type of intellectual exercise depends on a willingness to describe your opponents in terms which they would accept, and use themselves, and then explain why you disagree with them. Clegg instead offers a caricature of Labour, though one which would be widely shared among his own party.
Indeed his critique of Labour is somewhat Blairesque in style and perhaps rather too Cameroonian in content: it is essentially that Labour was right (and the Liberals wrong) about the importance of collectivism in the 20th century, but that the LibDems are right (and Labour wrong) about the failures of collectivism and the state in the 21st. So the central challenge to the Conservatives is often their allergy to multilateral cooperation.
As contemporary politics, this seems to me to be driven by a large dose of rhetoric and positioning. But I am not sure that this works as history either (though I don't claim to be an expert on developments in the academic debate, and would be interested to hear from those who might differ).
Firstly, the Attlee post-war settlement was in large part an achievement of Labour men and New Liberal as well as Fabian ideas, with the central role played particularly by Beveridge and Keynes. (It was the Liberals who had earlier pressed hardest for the interventionist state in Lloyd George and Keynes' campaigns on unemployment against economic orthodoxy in the 1920s and 1930s). Secondly, the Labour party did not have any official ideology before Sidney Webb's 1918 constitution. It could certainly be argued that ideology remained something of a minority pursuit in a primarily Labourist party afterwards.
So perhaps Liberalism's defeat was political, rather than ideological. The Liberal failure was perhaps less an aversion to collectivism (which the New Liberals had some response to) than their difficulty in representing rising political constituencies in the age of universal suffrage. In particular, the question of working-class representation rather than political ideology proved the most difficult hurdle.
By contrast, the eclipse of Liberalism to a fringe party between 1935 and 1975 saw middle-class progressive opinion play a significant role in the Labour alliance. So, perhaps counter-intuitively, the party which had been founded on a class ticket in fact proved a rather more enduring vehicle than the Liberals had for the cross-class progressive alliances which underpinned the 1945 landslide (as they had in 1906 albeit in a narrower electorate, and were to do so again to some extent in 1966 and 1997). The greatest liberal political and social advances after 1945 were overseen by Roy Jenkins as a Labour home secretary. Clegg charges Labour with "betrayal" over Iraq and civil liberties, yet it remains the case that New Labour's constitutional half-revolution with devolution and freedom of information amount to the deepest shift in British governance since 1911 too.
In 2009, it is rather difficult to find much evidence for a wholesale realignment of the type Clegg anticipates, though this may be very sensible positioning for a third party in an election year. James Graham's post also makes some cogent points about the barriers to this.
Do the LibDems today increasingly represent rising social movements and constituencies that Labour can not reach? Do their local government advances in the cities relly provide a base to leapfrog Labour by capturing the larger part of its core support? The Spring elections of 2008 and 2009 might well prove Labour's lowest ebb; the LibDems will have wanted to do better than 4th in the European elections. The party has had a good crisis with elite opinion; it has reaped little reward yet with the broader public. Nor is there much evidence for the 'swing left and civil war' return of the 1980s thesis, as Phillip Cowley's analysis of the political geography of the PLP suggests.
So both Labour and the LibDems have a political problem with the politics of support, building broad enough coalitions to govern. In response, Clegg proposes a death match for supremacy on the centre-left.
His aspiration to realignment is somewhat in the tradition familiar to activists in both parties, of wishing the distraction of the 'other lot' would disappear. But both parties will be with us for the foreseeable future: Labour's persistent levels of voter identification and its trade union and working-class base give it ballast which is underestimated; the expanded LibDem presence in local government and Parliament too is not going to evaporate too.
Clegg is right that the parties share, and contest, progressive space.
He is no doubt right as a matter of political reality that the politics of progressive cooperation are too difficult now.
We don't know how we might work together - and most, on both sides, would prefer not to. Still, it is an argument which will, and which should, return.
PS: Clegg is at least the second major politician to offer a detailed reading of Lab-Lib history in the last month. An alternative reading was offered in Jon Cruddas' Keir Hardie lecture, extracted in the New Statesman. Cruddas is now promoting cross-party dialogue between Labour, LibDems, Greens and others - and perhaps this might explain the sharpness of his comments on history, in a piece which mostly traces the complex interaction of liberal and socialist ideas.
Implied in the move to uncover and reconnect liberal traditions in our party is the view that the foundation of an independent Labour Party with a distinctively socialist outlook was a historic wrong turning, and that the progressive left would have been better off devoting its energies to building an enduring electoral base for a strong and reformed Liberal Party. This conclusion is not stated openly, but is implicit in much contemporary discussion. Hardie, however, would have been appalled. And so should we today.
On reading that a couple of weeks back, I couldn't help thinking that Dr Cruddas was rather massively overstating the interest within New Labour in history and historiography. But perhaps Nick Clegg's new pamphlet is a sign that history might be the new political rock and roll after all.