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Across countries, union membership and voter turnout are highly correlated. In unadjusted terms union members maintain a roughly 0.10 to 0.12 point gap in voting propensity over non-members. We propose a model -- with three causal channels -- that explains this correlation and then empirically tests for the contribution of each channel to the overall union voting gap. The first channel through which union members are more likely to vote is through the so-called "monopoly face" of unionism whereby unions increase wages for members and higher incomes are a significant positive determinant of voting. The second is the "social custom" model of unionism whereby co-worker peer pressure creates incentives for union members to vote alongside fellow members. The third channel is based on the "voice face" of unionism whereby employees who are (or have been) exposed to collective bargaining and union representation at the workplace are also more likely to increase their attachment to democratic engagement in society at large. We test to see how much of the raw "union voting gap" is accounted for by these three competing channels using data from 29 European countries. We find that all three channels are at work, with the voice accounting for half of the overall gap and the other two channels (monopoly face and social custom) each accounting for approximately a quarter of the overall union voting gap.