How do parties organize, and do parties' organizational differences matter? Different organization patterns are empirically associated with varying electoral performance, voters' participation, policy-making, and party systems' shape and stability.Despite the empirical relevance of party organization, theoretical scholarship has overwhelmingly focused on other functions of parties — namely the electoral one, simplifying the political world for voters, and the policy-making one in the legislative arena. The papers in this dissertation advance a new theoretical agenda on the organization of political parties, generating insights that I test with novel data. The main contribution of the dissertation is to treat party organization as an endogenous rather than exogenous variable. This approach allows to generate novel insights on how the electoral environment influences the way parties organize, and outcomes such as parties' electoral performance and the process of party system stabilization. The first paper conceives the internal organization of a party as being driven by factional competition. What brings opposing factions to engage in sabotage rather than enhance the party image, and what strategies can parties adopt to contain it? The paper introduces a model of elections in which intra-party factions can devote resources to campaign for the party or to undermine each other and obtain more power. The party redistributes electoral spoils among factions to motivate their investment in campaigning activities. The model shows that sabotage increases when the stakes of the election are low — e.g., in consensus democracies that grant power to the losing party — because the incentives to focus on the fight for internal power increase. It also suggests that the optimal party strategy for winning the election in the face of intra-party competition is to reward factions with high powered incentives when campaigning effort can be easily monitored, but treat factions equally otherwise. Finally, the model shows that, when a party weakens electorally, factions’ incentives move from campaigning for the party to sabotaging each other to obtain electoral spoils. A testable implication of this result is the emergence of political scandals triggered internally as a product of factional sabotage. The second paper tests this empirical implication using original data on judicial investigations of Italian MPs involved in various misbehaviors. Judicial investigations of politicians are a fundamental component of politics, often leading to scandals. Yet, empirical evidence of the strategic determinants of judicial investigations is intrinsically hard to gather, a problem that has significantly limited the study of this important phenomenon. The paper studies the politics behind judicial investigations leveraging new data on prosecutors' informants in 1125 episodes of misbehavior of Italian MPs involved in different crimes (1983-2019). Results provide evidence in favor of a political use of denunciations for corruption crimes: when a party weakens, the likelihood that political enemies denounce past misbehavior of members of the weakened party increases, suggesting that the political use of denunciation is elastic to changes in the electoral performance. The timing of past misbehavior is crucial: members of weakened parties are more likely to be accused of misbehavior that happened a long time before the accusation, which further supports the conjecture that accusations are politically motivated. The third paper moves to the topic of party organization in the presence of multi-party competition. It conceives of the choice over party organization as parties' decision to form different types of alliances. Despite being pervasive, little is known about the conditions facilitating different forms of pre-electoral alliances. The paper presents a model of electoral competition in which parties can form alliances before elections, and decide how binding these should be. Parties face a dynamic trade-off between insuring themselves against large shifts in public opinion and allowing flexibility to respond to future changes in voters' preferences. The model shows that more binding alliances such as mergers emerge in equilibrium when electoral volatility is high; otherwise, parties form more flexible pre-electoral coalitions. It also suggests that some power concentration is needed for alliances to emerge in equilibrium, whereas parties run alone under consensual democracies that share power among all parties.