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, Colorado; Chicago, Los Angeles, New York City, Philadelphia and South Miami, Florida, in the lawsuit.
Reynolds refused to consent, he said.
Her spokesman said Wednesday she supports Trump's policy.
"She does not believe it's in the best interest of Iowans for the state to join in this lawsuit," said Pat Garrett in a statement.
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.
Miller's spokesman Lynn Hicks said Miller was asked by the other states to join the case but couldn't when Reynolds didn't give consent.
Last year Miller supported the Clean Power Plan developed by the U.S. Environmental Protection Agency during President Barack Obama's presidency. It would require cutting emissions from fossil fuel-burning power plants. It was projected to prevent up to 3,600 additional deaths annually, said California Attorney General Xavier Becerra, one of the attorney's general filing the lawsuit.
In June, the EPA eliminated the Obama plan and replaced it with Trump's Affordable Clean Energy rule that gives states more leeway in deciding upgrades for coal-fired power plants.
White House spokesman Judd Deere said Trump's plan responsibly protects clean air, reduces greenhouse gases, protects jobs, and keeps costs affordable. He called Obama's rule burdensome and unlawful and said it would have raised energy costs for consumers.
Miller's spokesman Hicks said Trump's rule is a step backward.
"The rule fails to promote clean energy, but instead incentivizes the use of coal-fired power generation," he said, adding it would do nothing to address climate change harms caused by carbon pollution.
Ironically on Monday Reynolds signed a proclamation at the Iowa State Fair celebrating the state's progress on reducing reliance on coal energy and expanding wind power.
Iowa is second in the nation in installed wind energy capacity and is projected to produce 40 percent of its electricity from wind turbines by next year.
"Wind energy industry in Iowa saved 4.8 billion gallons of water, equivalent to 37 billion plastic water bottles, and avoided 8.8 million metric tons of carbon dioxide, equivalent to 1.9 million cars' worth of emissions, all in 2018 alone," Reynolds said.
Coal's share of net electricity generation in Iowa has declined from 76 percent in 2008 to 45 percent in 2017, Hick said. Progress that is endangered by Trump's policy, he said.
The cities and states sued the Trump administration in the U.S. Court of Appeals for the District of Columbia Circuit on Tuesday alleging it violates the federal Clean Air Act because it does not meaningfully replace power plants' greenhouse gas emissions.
The states joining the lawsuit are California, Colorado, Connecticut, Delaware, Hawaii, Maine, Maryland Massachusetts, Michigan, Minnesota, New Jersey, New Mexico, New York, North Carolina, Oregon, Pennsylvania, Rhode Island, Vermont, Virginia, Washington, Wisconsin and the District of Columbia.
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