Abstract Prior literature has shown strong and increasing polarization on environmental voting in the US Congress, with Democrats tending to support pro-environmental (“green”) legislation and Republicans opposing it. Employing a regression discontinuity design, we provide a causal estimate of the effect of partisanship on the importance of campaign contributions from brown industries received by US legislators. Electing a Republican rather than a Democrat for the House (Senate) leads to an increase of 96% (70%) in the importance of contributions from anti-environmental (“brown”) groups. We also find that greater dependence on brown groups is associated with less pro-environmental voting for legislators from both parties and the greater the importance of brown contributions the more (less) likely that a legislator breaks the party line when its party favors (opposes) pro-environmental legislation. Finally, we provide an analytical narrative of the rise and pattern of environmental polarization consistent with our empirical findings. (JEL D72, Q58)