The survival and termination of party mergers in Europe

Why do constituent parties that participated in a party merger that was intended to be permanent decide to leave the merger to re‐enter party competition separately? To address this question, merger termination is conceptualised in this article as an instance of new party formation, coalition termination and institutionalisation failure. Building on this conceptualisation, three sets of factors are presented that account for which mergers are likely to be terminated by constituent parties and which are not. To test these three sets of hypotheses, a mixed‐methods design is used. First, survival analysis is applied to a new dataset on the performance of mergers in 21 European democracies during the postwar period. The findings support hypotheses derived from a conception of merger termination as new party formation: pre‐ and post‐merger legislative performance significantly affect the probability of merger termination. Furthermore, the institutionalisation of constituent parties helps to sustain mergers if the latter already built trust in pre‐merger cooperation, in line with the conception of merger termination as institutionalisation failure. Two theory‐confirming case studies are then analysed: one case of merger survival and the other of termination. These case studies substantiate the working of the significant variables identified in the large‐N analysis that drove the selection of case studies. They also reveal how mediating factors difficult to capture in large‐N designs help to account for why factors that – theoretically – should have complicated the working of the ‘survival case’, and should have been beneficial to the ‘termination case’, did not generate the expected effects.