Since the 2016 election, President Trump has achieved unparalleled dominance over the Republican Party. He has also given his party a central role in his reelection campaign and invested heavily in its organizational capacities. This dual approach to party leadership – domination paired with organizational investment – bears a strong resemblance to the way every Republican president since Eisenhower interacted with his party, different only in degree. Where Trump’s party leadership diverges qualitatively from past patterns is in its apparent purposes. Previous Republican presidents dominated and invested in their party for the explicit purpose of building a new majority in American politics. Reaching out to new demographic groups and trying to persuade them to join the party was integral to this project. Trump, in contrast, has (thus far) predominantly pursued a base-mobilization strategy. Rather than fan out horizontally in search of new groups to join the party coalition, Trump’s strategy drills down vertically to penetrate and deepen his base. Instead of trying to diversify the GOP and extend its reach, his strategy aims to swell the number of like-minded supporters who are active in electoral and party politics (while suppressing, demobilizing, and delegitimizing the opposition party). By setting into motion a mutually reinforcing cycle of party domination and base mobilization, and amplifying its effects through organizational investment, Trump has turned his party into a formidable vehicle for advancing his personal purposes and augmenting his power – while raising troubling questions about the stability of American democracy. This article examines Trump’s party leadership to date, compares it to previous presidential party leadership projects, and considers the implications.