Phone calls encouraging citizens to vote are staples of modern campaigns. Insights from psychological science can make these calls dramatically more potent while also generating opportunities to expand psychological theory. We present a field experiment conducted during the 2008 presidential election (N = 287,228) showing that facilitating the formation of a voting plan (i.e., implementation intentions) can increase turnout by 4.1 percentage points among those contacted, but a standard encouragement call and self-prediction have no significant impact. Among single-eligible-voter households, the formation of a voting plan increased turnout among persons contacted by 9.1 percentage points, whereas those in multiple-eligible-voter households were unaffected by all scripts. Some situational factors may organically facilitate implementation-intentions formation more readily than others; We present data suggesting that this could explain the differential treatment effect that we found. We discuss implications for psychological and political science, and public interventions involving implementation-intentions formation.