The Obama administration is poised to rescind a little-known emissions exemption for power plants as it seeks to address climate change. Green groups are pressuring the Environmental Protection Agency to finalize a rule that would force utilities to limit emissions when power plants are shutting down, starting up or malfunctioning. Those emissions had previously been exempted from regulations because they were not considered a part of “normal operations,” but the EPA says that policy is now “outdated.” The environmental group Sierra Club held rallies last week near two facilities that would be affected by the end of the exemption and...