In the Union Electric Co. v. Environmental Protection Agency case of 1975, the U.S Supreme Court ruled in favor of the EPA, upholding its authority to enforce air quality standards under the Clean Air Act (CAA). The dispute began when Union Electric Company challenged an implementation plan set by Missouri and approved by EPA that aimed at achieving national ambient air quality standards for sulfur oxides within a stipulated timeframe. The company argued that such enforcement was beyond EPA's statutory power and it would cause economic hardship as they would need to convert their coal-fired plants into oil or gas-based ones which were expensive. However, rejecting these arguments, Justice Thurgood Marshall delivered majority opinion stating that Congress intended for states and localities to achieve clean air goals even if it required difficult choices about public policy priorities including costs associated with compliance.

In the dissenting opinion for Union Electric Co. v. Environmental Protection Agency, Justice Rehnquist disagreed with the majority's view that the Clean Air Act gave EPA authority to reject a state implementation plan on grounds of inadequacy without first giving states an opportunity to revise their plans. He believed this interpretation was inconsistent with Congress' intent and undermined federalism principles by allowing EPA to usurp state power in environmental regulation. Furthermore, he argued that such broad discretion given to EPA could lead to arbitrary decisions and potential abuse of power as there were no clear standards or guidelines provided by Congress regarding what constitutes an 'adequate' plan under the Act. Therefore, he felt it necessary for courts not only review whether procedural requirements have been met but also assess substantive reasonableness of agency actions within statutory limits set by Congress.