Parties, Strong And Very Strong

In parliamentary systems, a political party is considered strong if it is contained in every cabinet that is approved of by the legislative majority controlling the selection of all portfolios. A party is considered very strong when it fills every portfolio in the cabinets approved of by the legislative majority. Why this distinction? The difference between strong and very strong parties plays an important role in Michael Laver and Kenneth A. Shepsle's analysis of portfolio allocation in parliamentary cabinet formation. Say we are interested in explaining how a cabinet will be composed based on a preliminary distribution of party shares and policy preferences using majority rule. A strong party is a relevant actor in cabinet formation because it is the only such actor that can petition to form a government on its own. A very strong party is not only one that can insist on forming a government autonomously, but also its doing so is the only equilibrium solution to the cabinet formation problem under majority rule. Understanding these outcomes is helpful in deciding the possible composition of cabinets under different permutations of party system structure and issue space.

The findings from Laver and Shepsle's experiments from simulated parliamentary configurations is that strong parties are more likely in parliaments with low policy dimensions and low numbers of parties. Although less common than strong parties, the emergence of very strong parties is correlated to the same factors: parliamentary systems with low-dimensional issue space and party system structure. Interestingly, there can only ever be one strong party in a given iteration of Laver and Shepsle's simulated parliament.

Laver and Shepsle's account of strong parties contrasts with more standard game-theoretic accounts of core parties. The difference is that core parties exist within the win-set in unconstrained policy space. For Laver and Shepsle, strong parties exist within the win-set, but the win-set is constrained by the potential allocation of portfolios of parties that might form a coalition. While an important distinction theoretically, there is little empirical work that has attempted to distinguish the two conceptions.