The US government is one of the largest purchasers of vehicles, energy, and buildings in the world. This includes owning over 650,000 vehicles and 430,000 buildings. Green technologies are already advanced enough to be competitive in many sectors, but need large committed buyers to help producers realize their potential economies of scale.
Shifting government procurement policies aggressively towards green vehicles would not require Congressional action. Agency purchasing is well within the established scope of executive orders. The President could have immediate, massive impact on the adoption of green technologies without internecine political debates. Converting the federal fleets and retrofitting all governmental buildings would have the greatest impact on climate change to date.