Since reentering politics several years ago, Donald Trump has defined himself in opposition to a single figure: Barack Obama. He was the leading proponent of birtherism in 2011 and continued to promote the conspiracy theory publicly until 2015. (He reportedly has remained a birther in private.) Some believe Trump only decided to run for president because Obama made fun of him once. In declaring his candidacy, Trump described Obama as a “negative force” who was “not a leader.” And his campaign platform, such as it was, centered on undoing Obama’s legacy: He pledged to reverse Obama’s immigration policies, repeal Obamacare, and, in the words of running mate Mike Pence, undo “every single Obama executive order.”

He has governed as promised. “Trump is obsessed with Obama,” New York Times columnist Charles Blow wrote last year, noting that “much of what Trump has accomplished—and it hasn’t been much—has been to undo Obama’s accomplishments, like pulling out of the Trans-Pacific Partnership and the Paris climate agreement and reversing an Obama-era rule that helped prevent guns from being purchased by certain mentally ill people.” He concluded, “For Trump, the mark of being a successful president is the degree to which he can expunge Obama’s presidency.”

Two years into Trump’s presidency, however, it’s clear that he aspires to undo more than just the Obama era. Some of his most high-profile actions have sought to reverse decades of progress—by Democratic and Republican administrations alike—in protecting the environment, workers’ rights, and public health. If Trump succeeds, his own legacy won’t be defined by his erasure of Obama’s accomplishments so much as the destruction of the modern regulatory state as we know it.

Andrew Wheeler, the acting administrator of the Environmental Protection Agency, last week announced a plan to rewrite the Clean Water Rule, a 2015 regulation which ensured that small bodies of water like wetlands and streams would be protected by the Clean Water Act of 1972. But the new rule, if implemented, would do much more than just reverse an Obama-era protection. “This would be taking a sledgehammer to the Clean Water Act and rolling things back to a place we haven’t been since it was passed,” Blan Holman of the Southern Environmental Law Center told the Los Angeles Times.

Vetoed by President Richard Nixon but overridden by Congress, the Clean Water Act originally only gave federal protection to traditional “navigable” waters—i.e., water that can be navigated by boat. But the bodies of water covered by the law were expanded several times over the years. George H. W. Bush implemented a policy that broadened federal protection for more wetlands, and George W. Bush and Bill Clinton strengthened Clean Water Act protections even further.