Blocking Coalition

A blocking coalition comprises a group of voters that collectively can prevent the passage of a proposal. Thus, a blocking coalition exerts negative power. The term is often used in a narrower sense to denote a coalition that can prevent a measure from being passed but cannot muster sufficient votes to pass a proposal.

Whether blocking coalitions exist depends on the voting rule. Simple voting games without blocking coalitions are called strong. Under simple majority rule with an odd number of voters, a coalition is either winning or losing but cannot be blocking. In contrast, blocking coalitions can occur if a tie is possible or if a qualified majority is required. For example, the Council of the European Union decides on legislation using a qualified majority or unanimity. In the case of unanimity, any single member state can form a blocking coalition consisting just of itself. Under qualified majority, the number of possible blocking coalitions depends on the voting threshold and the distribution of votes. By looking at the relative frequency of possible blocking coalitions, one can estimate a priori how likely indecision is for any given combination of these two factors.

The possibility of a blocking coalition might be the result of procedural rules other than the voting rule. For example, in the United States, Senate decisions are made by simple majority. However, the rule that allows filibusters means that a higher voting threshold may be required to bring a bill to a vote in the first place.