Challenges to an EPA rule delaying a chemical safety regulation aimed at protecting emergency responders being argued March 16 could pose a test of the Trump administration’s push to roll back regulations.

Local and national public health and environmental groups, labor unions, and 11 states sued the Environmental Protection Agency, arguing the Administrator Scott Pruitt violated the Clean Air Act in delaying until 2019 an Obama-era rule for new response and planning requirements for high-risk chemical facilities. The...