The purpose of this research note is a critical revision of Sartori’s index of coalition potential. Sartori devised the coalition potential, together with the blackmail potential, in order to discriminate among the parties and to spot the relevant ones in any given party system. Although the blackmail potential was never operationalized, Sartori made the attempt with respect to the coalition potential and advanced a relative index. Nonetheless, Sartori’s index of coalition potential is not very useful because, in his own admission, it is not a measure of the coalition relevance of the parties but rather a measure of the fragmentation of governmental coalitions. It is, in other words, an ex post measure with no predictive capability. The mainstream of the theory of coalition points out that in any coalition game the potential of each actor is directly proportional to its weight, that is the coalition potential of any actor increases directly as its weight increases. On the other hand, the coalition potential of any actor is inversely proportional to its relative political distance from other members of any winning coalition, that is the more distant the parties are from each other the more costly is their cooperation in any winning coalition. Anchoring on these two standards of the coalition theory, the paper puts forward a conceptual schema for the analysis and the measure of the coalition potential of parties in a predictive way. Such a measure should be based on their weights (measured as % of parliamentary seats) and relative distance (measured as position distance on a cardinal space). The schema helps the identification of four types of parties, with high or low coalition potential, complementary parties, and blackmailing parties.
