Wisconsin Gov. Scott Walker (R) told President Obama that his landmark carbon rule for power plants is “unworkable” for the Badger State.

Walker, who is expected to announce a run for president next month, stopped short of saying that Wisconsin would not comply with the Environmental Protection Agency’s (EPA) climate change regulation.

But in a letter to Obama dated May 21, he expressed “deep concerns regarding our ability to develop a state plan to comply with the proposal,” and said it is “riddled with inaccuracies, questionable assumptions and deficiencies that make the development of a responsible state plan unworkable.”

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Walker has long been critical of the EPA’s rule, proposed in June 2014 with a goal of slashing the power sector’s carbon emissions 30 percent by 2030. The EPA plans to make the rule final in August, and then states will have a year to submit their compliance plans before the feds implement their own.

Walker's letter is one of the sharpest criticisms of the plan yet from a governor, although leaders from more than 30 states have said they oppose the rule.

Oklahoma Gov. Mary Fallin (R) ordered state agencies in April to ignore the rule and not comply with it, making Oklahoma the only state to clearly declare its intent to disregard the regulation.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE (R-Ky.), a top opponent of the proposed rule, has actively lobbied state governors to ignore it.

“Think twice before submitting a state plan — which could lock you in to federal enforcement and expose you to lawsuits — when the administration is standing on shaky legal ground and when, without your support, it won't be able to demonstrate the capacity to carry out such political extremism,” he wrote in March.

Walker has said very little about his stance on humanity's role in climate change, but environmentalists have accused him of prohibiting the state’s Board of Commissioners of Public Lands from doing any work related to climate.

He’s also signed a pledge never to support a tax on carbon dioxide emissions, organized by Charles and David Koch-backed group Americans for Prosperity.

Walker wrote letters to the Obama administration in August and December of 2014 saying the rule would cost Wisconsin $3.3 billion to $13.4 billion, due largely to shutting down coal-fired power plants, a figure that he repeated in the most recent letter.

Apart from the cost considerations and technical problems, Walker said, he is concerned that the EPA cannot legally enforce its rule under the Clean Air Act.

“Absent significant and meaningful changes in the final rule, it is difficult to envision how Wisconsin can responsibly construct a state plan that can comply with the requirements of the Clean Power Plan without ignoring our responsibility to ensure safe, affordable and reliable electricity for the people of Wisconsin,” he wrote.

Ellen Nowak, chairwoman of Wisconsin’s Public Service Commission and a Walker appointee, testified at the Senate Environment and Public Works Committee in March, saying, “we question the very foundation of this proposal.”

She also refused to say whether she believes that greenhouse gases from human activity are contributing to climate change.

Wisconsin is also among numerous Republican-led states suing the EPA to have the rule overturned, a lawsuit that is unlikely to be successful.

In response to the letter, EPA spokeswoman Liz Purchia fought back, saying the agency is working closely with states on the regulation.

“EPA's Clean Power Plan is built on a time-tested state-federal partnership in the Clean Air Act for EPA to establish public health goals while providing states important flexibility to design plans to meet their individual and unique needs,” she said in a statement.

The final rule coming out in August will take into account the “unprecedented” input on the rule, including 4.3 million comments received, Purchia said.

Read Walker's letter below:

Walker May 2015 Letter

This story was updated at 6:19 p.m.