Previous studies of electoral authoritarianism identified that manipulations demobilize opposition supporters. Yet, less is known about whether radicals are more prone to abstention than moderates in manipulated elections. To answer this question, we disentangle two mechanisms of demobilization effect—the efficacy mechanism and the electoral supply mechanism—that have different expectations on the turnout rate of radicals and moderates. Our research leverages the disqualification controversy in Hong Kong in 2016, after which radical candidates who advocate self-determination or independence were filtered out from the electoral market. Using both aggregate-level and individual-level data, our analysis shows that a substantive demobilization effect exists. Crucially, we find that radicals and moderates are demobilized to a similar extent, and the decreases in perceived electoral fairness and importance of voting are similar between the two factions. These findings suggest that the efficacy mechanism is a more plausible explanation of the demobilization effect. Overall, this study extends our understanding of voting behavior and political attitude of opposition supporters in face of autocratization.