This article investigates the conditions under which policymakers are likely to decisively accelerate socio-technical transitions. We develop a conceptual framework that combines insights from historical institutionalism and the Multi-Level Perspective to better understand the political dimension in transitions, focusing particularly on the mechanisms of political defection from incumbent regime to niche-innovation. We distinguish two ideal-type patterns, one where external (landscape) shocks create a ‘critical juncture’ and one where gradual feedbacks change the balance of power between niche-innovation and regime. We also identify more proximate conditions such as external pressures on policymakers (from business interests, mass publics, and technologies) and policy-internal developments (changes in problem definitions and access to institutional arrangements). We apply this framework to two historical case studies in which UK policymakers deliberately accelerated transitions: the transition from rail to road transport (1920–1970); and the transition from traditional mixed agriculture to specialised wheat agriculture (1920–1970). We analyse the conditions for major policy change in each case and draw more general conclusions. We also discuss implications for contemporary low-carbon transitions, observing that while some favourable conditions are in place, they do not yet meet all the prerequisites for political acceleration.