Voluntary cooperation is a way to deal with efficiency issues created by local government fragmentation without having to rely on politically contested mergers. One drawback of these agreements is its vulnerability to conflict among politicians with differing party allegiances. We investigate the effect of partisan differences among local political leaders on their propensity to cooperate using data from Spanish municipal associations. We rely on a new administrative database spanning more than three decades that provides information on all the municipalities joining or leaving an association each year and on the change in the scope of collaboration. In terms of methods, we use an event history analysis to study the decisions to enter and leave an association. To identify the effect of partisan alignment, we rely on a close-elections regression discontinuity design. We compare municipalities in which the party ruling in most of the potential cooperation partners marginally won the local election to municipalities where this party marginally lost that election. We also look at the effect of alignment on indicators of gridlock, such as the inability to pass the association’s budget on time, the delinquency in the municipality’s financial contribution to the association, or the shrinkage or expansion of the scope of collaboration. Our findings might have important implications for the understanding of the cooperation between municipalities and its interplay with party politics.