This article examines the macro–micro dynamics linking party membership with protest participation. We theorise that institutional and extra-institutional engagement are mutually reinforcing and that party membership has a positive effect on party activism. We examine key ideational and structural factors identified in the literature to analyse the relative importance of various factors for party members' involvement. We look at micro-macro-level linkages by examining macro-level contextual variables as well as the extent to which these features mediate the individual-level effects. Our results suggest that party members support social movements for a variety of ideational and structural reasons but that strategic reasons are also important. Moreover, we find that contexts marked by more open political opportunities close the gap in social movement activism between party members and non-members suggesting that contexts with higher public spending as well as crises could be capitalised on to engage the wider public into political activism.