The turnover of political parties is a key mechanism of renewal of electoral choices. We present an organizational ecological theory on party system change, predicting that party system saturation (i.e., the effective number of parties compared to the party system’s carrying capacity) differently affects the entry and exit of niche and mainstream parties from lower house elections. Pooled times-series analyses on 352 elections, 509 parties, and 21 established democracies demonstrate that party system saturation indeed increases the likelihood of exit of mainstream parties but not of niche parties. Strikingly, we also find that party system saturation increases the entry of mainstream parties. Hence, an important paradox arises since oversaturation negatively affects their survival chances.