Past the saturation point: Why voters switch from mainstream to niche parties and vice-versa

When do voters switch from mainstream to niche parties and vice-versa? To understand these switches, we focus on the saturation of the party system. We theorize that when a party system is oversaturated – i.e. when a higher effective number of parties contests elections than predicted based on socio-political contextual characteristics (the system's ‘carrying capacity’) – it becomes increasingly likely that: (1) mainstream party voters defect to niche parties; and (2) niche party voters refrain from switching to mainstream parties. Based on vote-switching patterns in 15 countries and 53 elections, we find that oversaturation increases shifts from mainstream to niche parties. Further analyses show that this holds for shifts from mainstream to radical left and right parties, but not for shifts to green parties. This has important consequences for research on vote switching, the electoral consequences of policy differentiation and the competition between niche and mainstream parties.