This section contains the nature of the case and procedural background.

Congress set up a federalism-based system of air pollution control. Under this cooperative federalism approach, both the Federal Government and the States play significant roles. The Federal Government sets air quality standards for pollutants. The States have the primary responsibility for determining how to meet those standards and regulating sources within their borders. Upwind States must prevent sources within their borders from emitting federally determined 'amounts' of pollution that travel across State lines and 'contribute significantly' to a downwind State's 'nonattainment' of federal air quality standards. That requirement is sometimes called the 'good neighbor' provision. D promulgated the Transport Rule, also known as the Cross-State Air Pollution Rule. It defines emissions reduction responsibilities for 28 upwind States based on those States' contributions to downwind States' air quality problems. The Rule limits emissions from upwind States' coal- and natural gas-fired power plants, among other sources. The Transport Rule targets two pollutants, sulfur dioxide (SO2) and nitrogen oxides (NOx). The statutory text grants D authority to require upwind States to reduce only their own significant contributions to a downwind State's nonattainment. Under the Rule, upwind States may be required to reduce emissions by more than their own significant contributions to a downwind State's nonattainment. Ps challenged this use of the good neighbor provision to impose massive emissions reduction requirements on upwind Ps without regard to the limits imposed by the statutory text. Ps appealed.

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