From early 20th century headlines to presidential tweets, immigration is described frequently in terms of waves, floods, and tides. Although usage of this inundation metaphor has been widely documented, its potential influence on immigration attitudes has not been assessed empirically. Building from conceptual metaphor theory’s claim that abstract ideas can be grounded in simpler, concrete concepts, we hypothesized that using the inundation metaphor to understand immigration contributes to support for a U.S.—Mexico border wall as a figurative means to block immigrants. Accordingly, social media posts supporting a border wall contained more inundation-metaphoric expressions than messages opposing a wall and messages opposing immigration without reference to a wall (Study 1; N = 4,067). Converging experimental tests show, when controlling for political attitudes, exposure to the inundation metaphor increases support for a border wall (Studies 2a and 2b; N = 737). These findings add to the growing body of evidence that political cognition and policy attitudes are partly motivated by metaphoric comparisons to concrete ideas that are irrelevant in a literal sense.