Relying on post-election surveys, this paper analyzes how class and union membership condition voters' abandonment of mainstream Left parties and the alternatives chosen by former mainstream-Left voters in the period 2001-14. Inspired by Przeworski and Sprague's Paper Stones (1986), our analysis shows that Left parties face a trade-off between working-class and middle-class support and that unionization renders workers more loyal to Left parties that mobilize middle-class support. Union membership increases the likelihood that working-class citizens who abandon the mainstream Left continue to vote. It also increases the likelihood that voters abandon the mainstream Left in favor of radical Left parties while it decreases the likelihood that they turn to the radical Right. Controlling for union membership, middle-class leavers are less likely to abstain from voting and less likely to vote for the radical Right than their working-class counterparts. Middle-class leavers are more likely to vote for Greens and for mainstream Center-Right parties.