Partisan polarisation is the increasing political separation between supporters, members, leaders, or organisations associated with different political parties. It can involve ideological divergence between parties, increasingly distinct partisan identities, greater differences in political attitudes between party supporters, or stronger alignment of political positions with party affiliation. The term is particularly prominent in political systems where partisan identity structures political competition. Partisan polarisation can therefore describe both differences between political parties as organisations and differences between citizens who identify with those parties. It is broader than simply saying that two parties disagree because ordinary party competition necessarily involves disagreement. Polarisation implies a pattern of increasing separation or alignment around partisan divisions.
Partisan polarisation can have ideological and affective components. Ideological partisan polarisation occurs when party supporters or party organisations become more separated in their policy preferences. Affective partisan polarisation occurs when supporters develop increasingly positive feelings towards their own party and increasingly negative feelings towards opposing parties. The distinction is important because partisan identities can become emotionally powerful even when policy differences do not increase proportionately.
Research on affective polarisation describes it as a tendency for partisans to like or trust members of their own party and dislike or distrust those associated with opposing parties. Thus partisan polarisation can refer to a combination of political disagreement and social identity, but researchers should specify which component is being measured.
Partisan polarisation can be driven by political sorting. Sorting occurs when people with particular ideological, demographic, cultural, geographic, religious, socioeconomic, or other identities become increasingly concentrated within particular parties. When several identities align with the same partisan division, party competition can become a broader social division rather than a disagreement over individual policies. Party leaders can also reinforce partisan differences through campaign strategies, political messaging, legislative conflict, and elite rhetoric. Media environments and social networks can provide additional channels through which partisan interpretations are repeated and strengthened. These mechanisms are context dependent, and their relative importance differs among countries with different party systems and political institutions.
Partisan polarisation can affect legislative behaviour. When parties become more distinct, elected representatives may have stronger incentives to vote consistently with their party, while cross party cooperation can become more difficult on issues that are strongly associated with partisan identity. Parliamentary party groups use mechanisms of coordination and discipline to maintain collective positions, but the strength of discipline differs across political systems. Partisan polarisation does not necessarily eliminate all cross party cooperation because legislators can still share interests, negotiate amendments, form coalitions, or cooperate on particular issues. The empirical question is therefore whether partisan divisions are becoming systematically stronger and more consequential, not simply whether parties sometimes vote differently.
Partisan polarisation can influence how citizens evaluate political institutions. Political trust may become conditional on which party controls government, with supporters of the governing party expressing greater confidence and supporters of opposition parties expressing lower confidence. Contemporary research explicitly identifies partisanship as an important factor in political trust and examines how polarisation can affect trust in government. OECD research similarly identifies partisan differences as important in explaining trust gaps. This means that institutional evaluations can become tied to partisan identity, potentially making perceptions of government more dependent on whether citizens regard the governing party as their own side or an opposing side.
Partisan polarisation is not the same as a two party system. A multiparty system can experience strong partisan polarisation among several parties, while a two party system can contain substantial internal ideological diversity. Likewise, partisan polarisation is not identical to ideological polarisation because party identities can divide people even where policy differences are relatively modest. It is also not identical to affective polarisation, although partisan polarisation frequently includes affective elements. Precise terminology is particularly important in multiparty systems because citizens can support one party while having varying degrees of sympathy or hostility towards several other parties. Research on Europe therefore faces measurement challenges that are less pronounced in simple two party models.
Partisan polarisation can influence electoral behaviour by making party identity more important in determining political choices. Stronger partisan identities can increase political engagement, strengthen campaign participation, encourage political discussion, and make voters more consistent in their electoral preferences. At the same time, strong partisan division can make compromise more difficult and can encourage citizens to interpret political information through partisan lenses. The relationship between polarisation and participation is not inherently one directional. Recent research on ideological polarisation shows that political separation can increase engagement in some circumstances while being associated with lower political trust. The same distinction applies to partisan polarisation: increased political mobilisation and increased institutional distrust can coexist.
Partisan polarisation is best analysed by specifying whether the unit of analysis is parties, politicians, voters, or broader partisan groups. Researchers can measure party positions through manifestos or legislative voting, voter differences through surveys, and partisan affect through evaluations of in parties and out parties. Longitudinal analysis is particularly important because polarisation is a process rather than merely a static difference.
A large difference between two parties at one moment demonstrates political distance but does not by itself demonstrate polarisation unless there is evidence of increasing separation or another appropriate definition of the term. Partisan polarisation can therefore be ideological, emotional, organisational, or behavioural, and accurate analysis should identify the specific form rather than using the term as a general synonym for political disagreement.
