As the Vietnam War concluded with the failure of US foreign policy, the so-called “Cold War consensus” collapsed in American politics and society. A significant number of lawmakers came to revisit their national security positions, and under these circumstances the Anti-ballistic Missile (ABM) bills came up in the 91st Congress (1969–70). The costly missile program quickly stirred a major controversy, particularly over a trade-off between guns (defense budget) and butter (welfare spending). This article examines how and why party rank-and-file members in US Congress stayed the course or shifted their positions during the ABM debates. The empirical findings suggest that representatives did not immediately abandon their national security preferences, but rather employed gradual position shifts in legislative processes. In addition, institutional conditions such as “in-party” and “party-out-of-power” hindered or helped legislators' position reversals. This case study of the “guns-or-butter” debates in 1969 and 1970 sheds light on how the representative system in America works in response to public discomfort, with lawmakers trying to fine-tune their individual policy positions and collective party reputations simultaneously.