This is something someone could examine empirically. I think we can use the argument developed by Dan Olson in “Why Do Small Religious Groups Have More Committed Members” (Review of Religious Research. 49(4): 353-378.) and apply it to political parties (or any organization). The argument that (religious) groups located in areas where their members are a smaller proportion of the population have more committed members. If we translate this to political parties, this means member commitment in relatively small parties should be stronger. Dan Olson argues that these groups have higher rates of leaving and joining, processes that select for more committed members. Congregations with higher membership turnover rates have current members that are more committed (they attend service, they give money). If we translate this to political parties, member commitment should be higher in parties with higher membership turnover.