We expand the sociological study of power struggles in labor markets to consider how the threat of monopsony (a form of buyer power) affects the size and strength of labor unions (a form of worker power). We focus specifically on the interplay of local union membership and the entry of Walmart Supercenters — a monopsonistic employer and the largest employer in the US. We study (1) whether greater worker power dissuades Walmart Supercenters from entering a local labor market, (2) whether a successful Supercenter entry subsequently erodes local union membership, and (3) whether unions provide a protective effect against declining economic outcomes after a successful Supercenter entry. Applying stacked difference-in-difference estimates using restricted-access Panel Study of Income Dynamics data, we find that Walmart Supercenters are less likely to enter a local labor market that has high levels of union membership, even when conditioning on attempted Walmart entries. When Walmart Supercenter openings do occur, union membership declines by an average of 4 percentage points in the subsequent 8 years, and is primarily channeled through declining union membership in retail. Remaining union members are not protected against Walmart’s downward pressure on wages; in fact, annual earnings among workers who were unionized pre-treatment decline faster than for non-union members after a Walmart Supercenter opens. Worker power can be effective at preventing a rise in buyer power, but conditional on increases in buyer power, worker power tends to decline both in terms of size and protective strength.