On 8 October 1977, Reginald Prentice, a former Labor MP who had occupied several ministerial posts in the preceding two decades, declared that he was crossing the floor of the House of Commons to join the Conservative party. This episode has often been regarded as an important moment in the sequence of events that led to Margaret Thatcher's victory in the 1979 general election, yet Geoff Horn's perceptive biography, Crossing the Floor: Reg Prentice and the crisis of British Social Democracy, is the first study to provide a detailed account of Prentice's defection and the broader realignments that it exposed.

Although the immediate events that led to Prentice's defection provide the pivot around which Horn's narrative turns, his account locates these events within a much broader historical context. Indeed it employs Prentice as a lens through which to view the rise and fall of the social democratic tradition that was ascendant in the two decades that succeeded the 1945 general election. Two arguments punctuate the study. First, Horn contends that Prentice's defection revealed a broader frailty within the model of social democracy that the Labor party constructed in the postwar period. This formation, he argues, was fraught with a significant contradiction. Its principal objective was to reconcile social equality with the existence of capitalism by redistributing the surplus generated by private enterprise. Yet its commitment to Laborist traditions prevented it from pursuing the sort of trade union reform that was necessary for such a strategy to be effective. “The tragedy of social democracy,” Horn notes, “was that the British economy required a major transformation in the structure and culture of industrial relations, but Labour was unable to engineer such a change” (7). Prentice was one of the first leading social democrats to acknowledge Labor's deficiency, and as the party moved leftwards in the 1970s, he arrived at the conclusion that the preservation of social democracy would require the creation of a new parliamentary grouping.

Second, Horn argues that the rigidity of Britain's political system halted the advance of a “neo-revisionist” formation that might have revitalized British social democracy in the early 1980s. In the mid-1970s, Prentice, alongside figures like John Mackintosh, had begun to construct a more pluralist model of social democracy that could reconcile the market with social equality. But the progress of this project was impaired by an inflexible two-party system and a Labor party that privileged unity over intellectual reinvention.

It could be questioned whether Prentice was a political weathervane whose trajectory exposed broader shifts in Britain's political development. After all, he was the only Labor MP to join the Conservatives, and his iconoclasm often alienated him from the leaderships of both parties. But Horn makes a strong case for seeing Prentice as a figure who personified Britain's postwar consensus. As well as believing in the virtues of evolutionary reform that was legitimated by democratic conduct, he was also suspicious of any body of ideas that followed from theoretical abstractions rather than empirical observation. His defection thus exposed a hollowing out of the ideological framework that had informed the postwar policy settlement. But if Horn's study does much to locate Prentice within a broader period, it might also have benefited from a wider frame of reference. Prentice may have been the only Labor MP to cross the floor of the House of Commons, but many other social democrats, including John Vaizey, Hugh Thomas, and Paul Johnson, made rightward turns following Thatcher's acquisition of the party leadership. And by locating Prentice in relation to them, Horn might have identified a more comprehensive formation that was bound together by a shared diagnosis of Britain's economic and social crises.

It would have also been profitable for Horn to explore the relationship between British social democracy and the Conservative tradition with which it competed. Prentice did not fit neatly within either camp, yet he was committed to beliefs and values that were shared by both. His story can thus be used to trace the permeable boundary that separated the ideologies of the Labor and Conservative parties. Yet Horn's account, despite making occasional references to Prentice's respect for the rule of law and his preference for evolutionary change, does not pursue this line of enquiry. These omissions do not detract from the overall quality of Horn's analysis; they do, however, suggest that more work needs to be done to explain the full significance of the political reconfigurations that his account describes.

As well as illuminating the Labor party's internal struggles, Horn also captures the turbulence and uncertainty that marked the intellectual politics of the late 1970s. By tracing Prentice's growing disillusionment with the orthodoxies of postwar Laborism, his work exposes the profound shock that the crises of the period exerted upon the intellectual foundations of the postwar consensus. And by untangling the reasoning that informed Prentice's decision to join the Conservatives, it reminds us that politicians of the center, as well as those of the left and right, were prepared to think the unthinkable in their efforts to resolve the problems that they observed. Prentice and other moderates may have been attempting to preserve a set of values and beliefs, but in doing so, they jettisoned many of their older assumptions and commitments.

Horn's study performs two important functions: it adds considerable depth to our understanding of the political reconfigurations of the 1970s, and by weaving Prentice into a much broader narrative about the fate of postwar social democracy, it reminds us that political biography remains a powerful historiographical tool.