Despite the administration’s desire to undercut fuel economy and emissions standards, the EPA will probably be thwarted by provisions of the Clean Air Act of 1970, amended in 1977 and 1990. The agency is required by the law to regulate emission of pollutants which "endanger public health and welfare," and it is estimated that since 1970 the Clean Air Act has prevented “more than 400,000 premature deaths and hundreds of millions of cases of respiratory and cardiovascular disease.”
Because the Clean Air Act prohibits states from enacting their own emissions standards for motor vehicles, states must seek waivers if they want stronger regulations. In July 2009, the EPA granted a waiver to California for its greenhouse gas emissions standards. (The California Air Resources Board originally requested the waiver in 2005.) The Clean Air Act also allows other states to adopt California’s emissions standards for which a waiver has been granted, and they are not required to seek EPA approval.
In May 2018, California, 16 other states and the District of Columbia filed a lawsuit against the EPA proposal. The states joining California are CT, DE, IL, IA, ME, MD, MA, MN, NJ, NY, OR, PA, RI, VT, VA, WA. The coalition is confident of winning because there is no provision in the Clean Air Act for revoking a waiver, and no precedent.