Donald Trump had barely finished announcing his pick to lead the Environmental Protection Agency before the left started listing its million reasons why Oklahoma Attorney General Scott Pruitt was the worst nomination in the history of the planet: He’s an untrained anti-environmentalist. He’s a polluter. He’s a fossil-fuel fanatic, a lobbyist-lover, a climate crazy.

Mr. Pruitt is not any of those things. Here’s what he in fact is, and the real reason the left is frustrated: He’s a constitutional scholar, a federalist (and a lawyer). And for those reasons he is a sublime choice to knock down the biggest conceit of the Obama era—arrogant, overweening (and illegal) Washington rule.

We’ve lived so many years under the Obama reign that many Americans forget we are a federal republic, composed of 50 states. There isn’t a major statute on the books that doesn’t recognize this reality and acknowledge that the states are partners with—and often superior to—the federal government. That is absolutely the case with major environmental statues, from the Clean Air Act to the Clean Water Act to the Safe Drinking Water Act.

Congress specifically understood in crafting each of these laws that one-size-fits all solutions were detrimental to the environment. Federal bodies like the Environmental Protection Agency traditionally and properly existed to set minimum standards, provide technical support, and engage in occasional enforcement. States, with their unique knowledge of local problems, economies and concerns, were free to innovate their own solutions.

But President Obama never held much with laws, because he failed at making them. After his first two years in office, he never could convince the Congress to pass another signature initiative. His response—and the enduring theme of his presidency—was therefore to ignore Congress and statutes, go around the partnership framework, and give his agencies authority to dictate policy from Washington. The states were demoted from partners to indentured servants. So too were any rival federal agencies that got in the EPA’s way. Example: The EPA’s pre-emptive veto of Alaska’s proposed Pebble Mine, in which it usurped Army Corps of Engineers authority.

One revealing illustration from EPA world. Under the Clean Air Act, states are allowed to craft their own implementation plans. If the EPA disapproves of a state plan, it is empowered to impose a federal one—one of the most aggressive actions the agency can take against a state, since it is the equivalent of a seizure of authority. In the entirety of the presidencies of George H.W. Bush,Bill Clinton and George W. Bush, the EPA imposed five federal implementation plans on states. By last count, the Obama administration has imposed at least 56.

Much of Mr. Pruitt’s tenure as Oklahoma’s AG was about trying to stuff federal agencies back into their legal boxes. Most of the press either never understood this, or never wanted to. When the media wrote about state lawsuits against ObamaCare or the Clean Power Plan or the Water of the United States rule, the suggestion usually was that this litigation was ideologically motivated, and a naked attempt to do what a Republican Congress could not—tank the president’s agenda.

The basis of nearly every one of these lawsuits was in fact violations of states’ constitutional and statutory rights—and it is why so many of the cases were successful. It was all a valiant attempt to force the federal government to follow the law. And it has been a singular Pruitt pursuit.

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It doesn’t need a climate warrior, as Congress has never passed a climate law, and so the EPA has no mandate to meddle there. What it needs is a lawyer, one with the knowledge of how to cut the agency back to its proper role—restoring not just an appropriate legal partnership with the states, but also with other federal bodies. One who reminds agency staff that the EPA was not created to oppose growth and development.

If Mr. Pruitt does this successfully, and on the way crushes the current president’s legacy, Mr. Obama will have only himself to blame. His abuse of federal power helped elect a new generation of state attorneys general and Washington Republicans passionately devoted to a states’ rights agenda. They’ll be advising Mr. Trump not just on environmental policy, but on health care, labor policy, entitlement reform. Say hello to the federalist revival.