While political parties in Europe are incredibly adaptable organizations, they have varied in the extent to which they are able to adapt to social and political transformations. I explain parties' adaptability in two steps. 1) Adaptability depends on factionalism in a nonlinear way. Giving too much room and no room at all to factions undermines a party's ability to adapt. 2) Factionalism depends on early organizational characteristics. The more centralized the initially introduced leadership selection process is, the more party elites will be incentivized to form factions. This argument applies to political parties that allow for internal competition and elect their leaders according to formal rules. I use statistical tools, a medium- and small-N analysis and systematic process tracing to test my framework against competing explanations. I focus on Christian democracy to use a most-similar system design. The main empirical part of the thesis relies on a structured focused comparison of the Italian DC, Austrian ÖVP and German CDU. It is guided by a nested analysis and builds on a large amount of primary data which has not been analyzed before. I test my theory on the additional cases of the Portuguese, Dutch and Luxembourgian Christian Democrats and the French MRP. My main finding is that early organizational choices matter. The initial form the leadership selection process takes has a decisive impact on the incentives of intra-party actors to form factions. The initial level of factionalism becomes deeply entrenched in the party's organization and internal code of practice. This explains why party elites are unlikely to change it when they realize that their party's level of factionalism undermines its adaptability. Moving beyond the focus of path dependence on a single level has thus important implications for the literature on party politics, factionalism, party organizations and institutional development.

L'objectif de cette thèse est la comparaison de deux partis politiques en France et en Grèce,de l'UMP et de la ND, tous les deux membres du PPE au parlement européen. Ils représentent dans les deux cas le centre droit et couvre la période de 2002 à 2009.Elle comprend quatre volets : celui de la création de l'UMP et de la ND ; celui de leur organisation ; leur volet idéologique, et ces deux partis face à des échéances électorales, en insistant à chaque fois sur leurs similitudes et leurs différences.En même temps, par le biais de ces deux partis politiques, elle permet la comparaison de deux régimes politiques : de la Ve République Française et de la IIIe République Hellénique, ainsi que les systèmes des élites en France et en Grèce. / The subject of this thesis is the comparison of two political parties in France and in Greece, the UMP and the ND, from 2002 until 2009, both members of the EEP in the European parliament and representing, in both cases, the centre-right.This comparison of similarities and differences between the UMP and the ND is based on four main issues: their creation, their organization, their ideology as well as some electoral campaigns.In the same time, the study of two political parties allows the comparison of two political regimes: the Fifth French Republic and the Third Hellenic one, as well as the elite systems in France and in Greece.