BACKGROUND: Understanding factors that influence public support for "nudging" policies, like pictorial cigarette pack warnings, may offer insight about how to increase such support. We sought to examine factors that influence smokers' support for requiring pictorial warnings on cigarette packs. METHODS: In 2014 and 2015, we randomly assigned 2149 adult US smokers to receive either pictorial warnings or text-only warnings on their cigarette packs for 4 weeks. The outcome examined in the current study was support for a policy requiring pictorial warnings on cigarette packs in the US. RESULTS: Support for pictorial warnings was high at baseline (mean: 3.2 out of 4). Exposure to pictorial warnings increased policy support at week 4 (β = .05, p = .03). This effect was explained by increases in perceived message effectiveness (p < .001) and reported conversations about policy support (p < .001). Message reactance (i.e., an oppositional reaction to the warning) partially diminished the impact of pictorial warnings on policy support (p < .001). CONCLUSIONS: Exposing people to a new policy through implementation could increase public support for that policy by increasing perceived effectiveness and by prompting conversations about the policy. Reactance may partially weaken the effect of policy exposure on public support.