The Northern Irish party system is plausibly represented as two systems, in which partycompetition occurs within nationalist and unionist blocs. Social and ideological divisions withinthese blocs constrain parties' electoral strategies and thus facilitate or inhibit cross-communalcompromise. Using Northern Irish Social Attitudes data, various accounts of these intra-communal divisions are tested and their political implications assessed. Contrary to expectationsconcerning the effects of cross-communal contact, neighbourhood integration is found to haveno relationship with partisanship although, consistent with political socialization theory, youngpeople on both sides of the divide are more likely than those who are older to support the morerecently introduced parties. Most significantly, however, there are noticeable asymmetries in thepatterns of cleavage within the unionist and nationalist blocs. Among Protestants, left–rightideology has a far stronger impact than constitutional position on patterns of partisanship; andsocial class has considerably stronger effects than does denomination. Most Protestants, whatevertheir partisanship, also express strongly unionist constitutional preferences. In clear contrast, onthe nationalist side party support is polarized along constitutional lines, there is no cross-cuttingideological division over economic inequality, and a majority of Catholics adopt a moderatestance on nationalism. It is argued in consequence, that within the unionist bloc the pattern ofintra-communal party competition militates against constitutional compromise as a solution to‘the troubles’, whereas among nationalists the unidimensional structure of competition forelectoral support and the distribution of attitudes towards the constitutional issue are likely to haveinfluenced the adoption of compromise strategies by Sinn Féin.