Changes in activism among grassroots members of the Labour and Conservative parties in Britain are modelled using panel surveys in this article, with party members being surveyed before and after the 1992 general election. The evidence suggests that a decline in activism over time has occurred in both parties, but this decline is greater in the Labour party than in the Conservative party. This is attributed to a number of political, cultural and sociological changes in society over time, but in the short run the main factor is the outcome of the general election in 1992. These trends are modelled by means of a ‘general incentives’ theory of activism, which explains the decline in activism in terms of changing incentives for political action. Reductions in incentives for activism were, with one exception, greater for Labour party members than for Conservatives, which explains the differences between the two parties. This may produce a ‘spiral of demobilization’ in which electoral losses produce a decline in activism and campaigning at the local level, which in turn leads to further electoral losses.