Cooperation between environmentalists, scientists, and governmental actors was a crucial driver behind Brazil's efforts to reduce greenhouse gas (GHG) emissions from the forest sector from 2004 onward. The same climate coalition's advocacy work to reduce emissions in the energy sector, Brazil's second‐most emitting sector, has been unsuccessful. Why has climate‐policy development been so different in the two sectors? Building on the advocacy coalition framework, this paper analyzes the climate coalition's role, systematically comparing how the coalition has worked to influence policy development in the two key sectors for Brazilian GHG emissions between 2003 and 2015. The paper finds that strong climate‐coalition unity, unambiguous scientific knowledge, economic growth, and international pressure functioned as a constellation of factors that enhanced the climate coalition's ability to take advantage of a climate‐policy window and frame deforestation as a core climate concern. The same constellation of factors was missing in the energy sector.