The normalization of radical politics has had decisive transformative impact on Western party systems over the course of the past 50 years. Radical parties, which typically campaign on messages initially considered outside the range of acceptability, have increasingly cast off the stigma of societal and political ostracism and have come to enjoy sizable and overt electoral support. In this paper, we investigate the mechanisms underlying this process of normalization and legitimization. We argue that the initial exceedance of expected electoral support legitimizes radical parties in the eyes of their supporters. As a result, voters will be more likely to disclose their support for radical parties and treat them as viable alternatives to established parties. Specifically, we hypothesize that radical parties’ electoral overperformance relative to the average pre-election polls sends a strong signal of increasing legitimacy, which will lead to lower levels of underreporting in vote choice for these parties and higher levels of disclosed vote switching from established mainstream parties to radical challengers parties in post-election surveys. These mechanisms of dynamic normalization will be most pronounced when radical parties first break through into national party systems. We test our arguments using a newly compiled data set that marries a large collection of post-election surveys with pre-election polls and official election results.