Donald J. Trump's 2016 presidential election victory spurred strong reactions and unprecedented collective action in the American Left. Taking advantage of the political climate in the wake of the election, this study examined whether the main antecedents of collective action (anger, political identification, and efficacy beliefs) in the immediate aftermath of the election loss for the American Left predicted varying types of collective action and social movement identification one month into Trump's presidency, and whether these factors in turn fuel anger and influence efficacy beliefs. Data collected from 913 self-identified liberal Clinton supporters at two time points (respectively, 7-10 days following the election and one month into Trump's presidency) revealed that political identification, anger, and efficacy to oppose Trump at Time 1 predicted engagement in collective action during the first month of Trump's presidency as well as higher identification with the emerging movement. While efficacy to oppose Trump predicted higher social movement identification, efficacy to change hearts and minds predicted lower social movement identification. We also examined the iterative processes of collective action, showing that the anger route was more central to galvanizing collective action than the efficacy route. These findings extend collective action research to contexts of emerging social movements following electoral processes.