How do parties become established at the national level? Existing studies focus primarily on country-level factors like institutional settings, but this paper argues that internal party characteristics—such as authority centralization, ideological coherence, and leadership factionalism—also play a crucial role. We employ new data covering 142 European political parties' organizational structures to provide empirical evidence for these dynamics.

Using original datasets collected from twenty countries across Europe, we conduct multilevel analysis that disentangles the effects of institutional context versus party-level factors on nationalization processes. This approach offers greater nuance than previous research by isolating specific organizational variables and their impacts.

Our findings reveal:

• Degree of ideological unity significantly accelerates parties' path to full nationalization across Europe
• Centralized authority structures are associated with faster entry into the national political arena, but not necessarily sustainable institutional establishment
• Leadership factionalism tends to hinder complete nationalization despite facilitating initial expansion in some contexts

These insights advance our understanding of party development by emphasizing internal organizational strategies as complements to external conditions.