While Joe Biden is known for his bipartisan deal-making, he must overcome a potentially more significant roadblock to fulfill his pledge of passing aggressive climate change legislation: winning over centrists from fossil-fuel states in his own party.

“If the Democrats gain control of the Senate, it’s going to be because a lot of purple states turned from red to blue, which suggests there will be less of a mandate to pass something wildly outside a moderate climate objective,” said Sasha Mackler, director of the Energy Project at the Bipartisan Policy Center.