Autocrats rely on co-optation to limit opposition mobilization and remain in power. Yet not all opposition parties that pose a threat to their regime are successfully co-opted. This article provides a formal model to show that reliance on activists influences whether an opposition leader receives and accepts co-optation offers from an autocrat. Activists strengthen a party’s mobilization efforts, yet become disaffected when their leader acquiesces to the regime. This dynamic undermines the co-optation of parties with a strong activist base, particularly those with unitary leadership. Activists have less influence over elite negotiations in parties with divided leadership, which can promote collusion with the regime. The results ultimately suggest that party activism can erode authoritarian control, but may encourage wasteful conflicts with the government.