In most democratic legislatures it is common for elected members of parliament to switch to existing or new parties or become independents. In this study we address the under-researched question of whether and when party switching by legislators affects party support in elections under the proportional electoral systems. We focus on the support of parties that lost MPs (origin parties) and those that received in-switchers (recipient parties). We focus on two sets of mechanisms through which party switching may affect voters’ party choice. First, the origin party may lose votes because switching potentially sends a strong signal to the electorate that the party is divided. Second, research on personalised politics implies that at least some voters follow switchers from origin parties to recipient parties. We expect that these effects are stronger when MP switches into an existing party (defection) or establishes a new party (split) than when MP becomes an independent in parliament (exit). We further expect that electoral gains and losses are concentrated in the electoral districts of the switcher MPs and the effect of switching on electoral outcomes is stronger when switchers are electorally prominent. Our empirical analysis uses data from two cases with closed-list PR systems (Italy and Romania) and one case with open-list PR (Poland) and combines district-level electoral data and individual voter data. Our study reveals how voters respond to switching and the potential of party instability in parliaments to lead to a broader party system change.