The 2017 GOP drive to “repeal and replace” the Affordable Care Act (ACA) arguably constituted the most ambitious effort to dismantle a social program in American history. Certainly it was the most ambitious to come so close to enactment, falling just three votes short in the Senate. According to an extensive body of scholarship, this near miss should have been nearly impossible. The political fallout associated with dismantling social programs, as well as their entrenchment in social and economic life, should deter electorally sensitive politicians from undertaking retrenchment of the scale Republicans sought. To unravel this mystery, we explain how the radicalization of the GOP is rooted in distinct electoral and organizational realities that have simultaneously increased the party's desire to move right and its capacity to do so—even when the policies that Republicans seek to enact are distinctly unpopular. We also explain why the ACA was underentrenched and why Republicans believed they could use legislative procedures and policy design to minimize the risks they faced. Finally, we highlight some new barriers to retrenchment in this age of “asymmetric polarization.” The hurdles facing retrenchment—in particular, the sway of national public opinion—have not simply diminished; they have changed, in ways that should be incorporated into our theories.