This article explores the strength and causal determinants of ideological thinking within Swiss local political parties. The concept of “ideologization” refers to (1): “horizontal couplings”, as they are manifested in intercorrelations between different opinions, and to (2) “vertical couplings” of specific opinions to abstract concepts of “left” and “right”. Results show high ideologization on the left-center section of the LR-scale, especially in the vertical dimension. On both sides of the spectrum, ideological constraints are significantly higher in larger communities than in than in smaller ones. Only in rather small communities, does ideologization correlate positively with the educational level, the modern occupational background of party members and the number of other local parties with which they have to compete. In communities of given size, ideological thinking is more pronounced when parties possess a small share of political power. Finally, it is found that ideological constraints have increased somewhat between 1989 and 2002.