The history of social democracy is likely an elusive, shape-shifting subject in every country but surely nowhere more so than in the United States. Except for a brief period (1900–20) when the Socialist Party of America – itself a diverse factional coalition – commanded at least minor attention on the national stage and again in recent years (2016–20) when Senator Bernie Sanders sallied forth as a ‘democratic socialist’ presidential candidate, no self-identified social democratic movement has much claim on our historical curiosity. Merely to stick within such obvious precincts, however, would likely reproduce a narrow rehash of factional politics (already covered by a competent specialized historiography) probably of as little interest to the readers of this volume as to its author. If we look below the surface, however, at the essence, rather than the form, of the matter, we will see that something akin to the social democratic idea as centred in Europe has also long inhabited American politics and political culture, at times with quite significant, if never yet triumphant, effect.