<p>Understanding when and why people accept electoral outcomes is vital to the study of political psychology, mass politics, and democratic governance. Despite its clear theoretical and practical importance, however, most existing research on electoral legitimacy have been cross-sectional or have only examined a narrow range of theoretically relevant factors. In this data brief, we present data for two independent panel studies. Sample 1 completed two-waves before/after the 2020 U.S. Presidential Election (Wave 1: <i>N</i> = 1,079, Wave 2: <i>N</i> = 903). Sample 2 approximated national representativeness and completed four-waves before/after the 2020 U.S. Presidential Election and before/after the 2022 U.S. Congressional Midterm Elections (W1: <i>N</i> = 1,127, W2: <i>N</i> = 769, W3: <i>N</i> = 506, W4: <i>N</i> = 453). Both surveys include a wide range of measures theoretically relevant to perceived electoral illegitimacy and political attitudes and behavior. These data enable the scientific study of critical questions that surround perceived electoral legitimacy in American politics.</p>