... Republican strategists say that rejection of Mr. Obama’s climate policy at the state level could emerge as a conservative litmus test in the 2016 election. ... Last year, the Environmental Protection Agency unveiled a draft regulation that stands at the heart of the president’s efforts to fight global warming. The proposed rule assigns each state a level by which it must reduce its planet-warming carbon emissions from electric power plants. Under the rule, which the administration expects to release in its final form in August, every state will have one year to draft a customized plan detailing how it will comply. States, for example, could submit plans to shut down heavily polluting coal plants, replacing them with natural gas plants and wind, solar and nuclear power generators, and to improve energy efficiency in buildings. They could also enact taxes on carbon pollution, or join regional “cap and trade” programs, which require companies to pay for government-issued pollution permits. The White House envisions the plan as a trigger that will prompt a transformation of the American electricity system, shifting it from dependence on fossil fuels to a reliance on renewable and low-carbon energy sources. But some governors call the proposal a federal intrusion on their authority. ...