What determines the failure of local government amalgamation referenda? Existing research suggests that functional pressures act as a push factor towards local territorial reform, whereas considerations of political self-determination exert a pull effect. However, we know little about the respective importance of these countervailing forces. In this paper, I analyze popular vote decisions on mergers of 541 municipalities involved in 166 different merger projects in three Swiss cantons since the new millennium. The results show that both functional pressures and concerns for self-determination are linked to popular vote outcomes: small municipalities are less likely to reject a merger. Concerns for self-determination matter, but only when the pressures of smallness are not overwhelming: a higher vote share of right-wing parties and a preponderance of other municipalities in the merger coalition increase the probability that voters reject a merger project. This has implications for policy-makers’ strategies when drafting and promoting voluntary local amalgamation reforms.