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.”