Whereas many advanced democracies have a long-standing tradition of collaboration between parties and interest groups, it is still contested what drives such collaboration. Linking data on political parties with survey data from over 750 Danish and Dutch interest groups we find evidence of groups focusing on collaboration with large and ideologically moderate parties in both systems. However, our findings also indicate that the importance of power and ideology for interest group-party collaboration is conditioned by crucial aspects of the institutional context in which such collaboration occurs related to party system dynamics and coalition governance. In Denmark, where governments tend to alternate between left and right, collaboration between parties and interest groups is more likely to follow a similar left-right division. In contrast, such collaboration is more likely to reflect a division between core and marginal parties in the Netherlands, where change in government composition is typically only partial.