The standard model of political party density emphasizing the interaction of social cleavages and district magnitude (M) is incomplete in accounting for number of parties in cases of high values of M in an arbitrary way. We explore an alternative model for such cases emphasizing the slack in the issue agenda available to parties with which to construct viable identities or niches they can employ to mobilize cognitively-limited voters. The model is tested with time series data and event history analysis on the sizes of the public policy agenda and the political party system in the Netherlands, an extreme case of large district magnitude given its single national district. Change in the slack of the issue agenda influences the births and deaths of political parties, a result that may also have implications for cases of lower district magnitude.