Party politics is about cooperation and conflict between political parties. At certain times, a party may rule out all political cooperation with a particular other party. In these cases, there is only conflict between the parties. This dissertation is about such situations. I define the systematic refusal of a particular party to cooperate politically with a particular other party as the party’s ostracism of the other party. It is the causes and consequences of ostracism that I wish to explain in this thesis. In order to do so, I first define the concepts of the ‘anti-immigration party’ and the ‘communist party.’ Based on both a literature review and an expert survey, conducted in the course of this study, I classify other parties’ responses to the existence of 46 anti-immigration and communist parties in 15 countries in postwar Western Europe as either ‘ostracism’ or ‘no ostracism.’ In the first part of this dissertation, I use the resulting classification as the dependent variable in a comparative-empirical analysis, in order to explain the variation in other parties’ political responses to particular political parties. Using logistic regression analysis on the basis of two different data sets, I find that other parties are likely to systematically boycott a far right party if they do not need to cooperate with it anyway. They are even more likely to do so if it holds anti-democratic ideologies. The ‘ostracism’ / ‘no ostracism’ classification is the main independent variable in the second part of the research, which aims at exploring the consequences of the exclusion of political parties for their electoral support. I argue and empirically demonstrate, by way of different regression analysis techniques on the basis of 15 different data sets, that when ostracized, anti-immigration and communist parties are less able to affect policy outcomes, which is what generally interests voters. As a result, these parties lose votes. There is one main exception to this rule. Anti-immigration parties that operate in a political context where opposition influence in parliament is high, and which are represented in the national parliament, are immune to the deleterious impact of ostracism on their electoral support. It seems that these parties can exercise power over policy-making in spite of being ostracized, thereby remaining attractive to voters. Thus, institutional factors determine whether or not ostracizing a rival party is an effective tool in the hands of targeting parties in order to safeguard democracy - or just to hold on to power.