At the conclusion of his presidency, Donald Trump’s political trajectory gave rise to three compelling questions: how did a businessman-turned-entertainer, with no prior political experience, succeed in securing the Republican nomination for the presidency of the United States? How can the relationship between Trump and the Republican Party be characterized, and how can it be understood? Finally, how are these relationships likely to evolve in the medium and long term following his tumultuous departure from office? Starting with an analysis of the immediate causes behind Trump’s victory in the race for the Republican nomination, this chapter shifts its focus to the peculiarities of the American party system that enable an outsider like him to successfully challenge the party establishment. Highlighting how the system’s rules provide an elected president with powerful tools to reshape internal party dynamics, this chapter examines how Trump leveraged the support of an external political movement to strengthen these mechanisms and marginalize his opponents. Based on this analysis, this chapter concludes that even though Trump’s personal fate was uncertain at the time, American society would probably live with its legacy for a long time.