Donald Trump’s 2017–2021 presidency and his challenge to the peaceful transition of power reflects a profound transformation within the Republican Party and raises questions about the evolution of conservatism into a form of authoritarianism. This chapter situates Trump’s first presidency in a century-long development of executive-centered partisanship, and details the institutional and partisan logics that allowed his administration to advance his “America First” agenda and reshape the GOP. Despite few legislative successes and endangering fellow Republicans’ electoral chances, Trump’s deep connection with the party base and his redefinition of party politics signify more than mere personality cult. This chapter argues that Trump’s approach represents a culmination of trends that threaten liberal democracy, suggesting that the challenges of presidential partisanship will persist beyond his first term and into his second period in power.