DES MOINES, Iowa — Iowa Attorney General Tom Miller, a supporter of clean energy policies reducing greenhouse gases as a way to battle global climate change, said Wednesday he was blocked by Republican Gov. Kim Reynolds from joining 21 other states in suing the Trump administration over a policy that relaxes restrictions on coal-fired power plants.

Miller, a Democrat, said he asked Reynolds in July if he could join attorneys general in the other states and officials from the cities of Boulder, Colo.; Chicago, Los Angeles, New York City, Philadelphia and South Miami, Fla., in the lawsuit.

Reynolds refused to consent, he said.

Miller reached an agreement with Reynolds in May requiring him to get the governor’s consent before joining such lawsuits. In exchange, Reynolds vetoed a bill lawmakers passed that would have weakened the attorney general’s powers to file out-of-state lawsuits. If signed into law, Iowa would have been the only state with such limits on the attorney general’s powers.