This article examines how challenger parties enter the political arena and the effect of this entry by looking at the Italian 5 Star Movement (Movimento 5 Stelle – M5S). We explain the M5S's entry strategy in 2013 using the spatial approach to party competition and employing expert survey data collected for each national election between 2008 and 2018. These data allow us to analyse the changing spatial configuration of Italian politics due to the increasing salience of pro/anti-EU and pro/anti-immigration dimensions. We then apply the theoretical notion of the uncovered set (UCS) to trace how the M5S's entry reshaped the overall space of party competition, causing a realignment of existing parties. This work contributes to the ongoing debate on the electoral success of challenger parties and the emerging cleavages and polarization of party systems in Western European countries.

This article promotes a characterization of intraparty politics that explains how rank- and-file party members control the delegation of power to their cabinet ministers and shadow cabinet ministers. Using the uncovered set as a solution concept and a measure of party members' collective preferences, we explore the hypothesis that backbenchers' preferences constrain the ministerial selection process in a manner that mitigates agency problems. Specifically, promotion is distributed preferentially to members whose own policy preferences are proximate to the uncovered set of all party members' preferences. Our analysis of ministerial appointments in the contemporary British Parliament supports this view. For both the Labour and Conservative parties, front bench appointments are more sensitive to the collective preferences of backbenchers in each party as measured by the party uncovered set than to the preferences of the parties' leaders.

Our investigation of the Senate politics of four major civil rights acts indicates that they did not result from winning coalitions bulldozing helpless minorities, nor did they result from some unpredictable chaotic process. These critical bills were the result of a flexible, multidimensional coalition-building process that proceeded by offering amendments carefully constructed to split off pivotal members of the winning coalition. Ideal point estimates of U.S. senators reveal that this coalitional negotiation process led to outcomes at some distance from the first choice of the winning coalition, testimony to significant compromise, both in early proposals and in refinements. This negotiation process resulted in outcomes apparently constrained by the boundaries of the uncovered set (McKelvey 1986; Miller 1980). “Closing the deal” in the U.S. Senate meant finding an outcome that could withstand robust attacks on pivotal coalition members—and that meant finding an outcome in the uncovered set.

The uncovered set has frequently been proposed as a solution concept for majority rule settings. This paper tests this proposition using a new technique for estimating uncovered sets and a series of experiments, including five-player computer-mediated experiments and 35-player paper-format experiments. The results support the theoretic appeal of the uncovered set. Outcomes overwhelmingly lie in or near the uncovered set. Furthermore, when preferences shift, outcomes track the uncovered set. Although outcomes tend to occur within the uncovered set, they are not necessarily stable; majority dominance relationships still produce instability, albeit constrained by the uncovered set.