This article uses new quantitative data to systematically test established arguments concerning the origins of party strength in 11 West European countries between 1870 and 1939. Through time-series cross-sectional analyses and using a wide range of sources, I investigate the role of the development of parliamentarism, suffrage extension, civil society, and the adoption of proportional representation. I further theorize that the development of parliamentarism and suffrage had an interactive effect because the legislative strength of a party affected its electoral strength, and vice versa. The main findings are that the development of parliamentarism most consistently helps explain party strength and that suffrage extension also was important, and I find support for an interactive effect of the two. A qualitative illustration of the Dutch development offers further support. Thus, the study provides a general explanation for the historical origins of a key component of West European parliamentary democracy, its strong parties.
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