This piece is part of a series on Obama’s legacy that The Huffington Post will be publishing over the next week.

WASHINGTON ― President Barack Obama could not convince Congress to send him new legislation to combat greenhouse gas emissions. But his legacy of fighting climate change might have more staying power because of that.

Congressional Democrats did try to pass a climate bill. The American Clean Energy and Security Act, from Reps. Henry Waxman (D-Calif.) and Ed Markey (D-Mass.), passed the House on June 26, 2009 after a brutal fight. The Waxman-Markey bill would have put an economy-wide limit on carbon pollution and created a system for buying and selling pollution credits — a policy called cap-and-trade. But the legislation sputtered out in the Senate, where even many Democrats showed little enthusiasm for it.

There was plenty of finger-pointing at the time — at Obama for not pushing harder on the bill or giving it the same priority as the health care package, at Senate Democrats for not trying harder, at the Senate Republicans who claimed to care about the issue but abandoned it once the bill became a reality.

The Obama administration always maintained its preference for Congress to pass a new, carbon-specific law. Doing so would “avert a regulatory thicket where governments and businesses spend an inordinate amount of time fighting,” said Obama’s first Environmental Protection Agency chief, Lisa Jackson.

But Obama always had a Plan B — acting on carbon emissions under the Clean Air Act. The administration began developing a regulatory approach immediately, a decision that proved far-sighted.

“Certainly it would be more permanent if we had a law in place,” Waxman, who retired in 2014, told The Huffington Post in an interview last month. “But it became obvious in 2009 and 2010 when the Senate was not able to pass a bill, Congress could not act, and that the Republican majority that took over in the next election was not interested in moving legislation. “

In Massachusetts v. Environmental Protection Agency, Massachusetts, environmentalists and a group of other states laid the groundwork for the Obama administration’s action by petitioning and eventually suing the George W. Bush EPA so it would regulate greenhouse gas emissions from motor vehicles under the Clean Air Act. The Supreme Court ruled in April 2007 that the agency is obligated to regulate any type of air pollution that “may reasonably be anticipated to endanger public health or welfare.”

By contrast, the Obama administration took up the issue almost immediately, announcing in April 2009 that carbon emissions pose a human health hazard. The Clean Air Act targets mobile pollution sources first, so the administration was already hard at work addressing auto emissions through an unprecedented collaboration between the EPA, the Department of Transportation, automobile manufacturers and progressive states like California. Announced in May 2009, those rules set the first-ever national limits on greenhouse gas emissions from cars and trucks.

For the sake of our children and the health and safety of all Americans, I’m directing the Environmental Protection Agency to put an end to the limitless dumping of carbon pollution from our power plants. Barack Obama, June 2013

Then the climate bill failed, and Republicans reclaimed the House in the 2010 midterm elections. Obama won a second term in 2012, but still faced a divided Congress. In June 2013, he turned his sights to administrative action on power plants — responsible for 40 percent of U.S. emissions at that time — with an impassioned speech at Georgetown University.

“We limit the amount of toxic chemicals like mercury and sulfur and arsenic in our air or our water, but power plants can still dump unlimited amounts of carbon pollution into the air for free. That’s not right, that’s not safe, and it needs to stop,” Obama said. “So today, for the sake of our children and the health and safety of all Americans, I’m directing the Environmental Protection Agency to put an end to the limitless dumping of carbon pollution from our power plants, and complete new pollution standards for both new and existing power plants.”

In 2015, the EPA announced rules for new and existing plants that would cut power plant emissions 32 percent by 2030, which it called the Clean Power Plan.

The incoming Donald Trump administration has pledged to undo those rules, and the Supreme Court has stayed their implementation while the cases questioning their constitutionali ty play out. Those cases will almost certainly end up before the Supreme Court in the end. Although the fate of the rules is uncertain, the fact that they were constructed under the Clean Air Act — a popular, bipartisan law with decades of precedent — and that their impetus originated in the Supreme Court might actually make them harder for Trump to dismantle than a new law would have been.

“There are hot dogs among the climate deniers who talk bravely about revoking the endangerment determination,” said David Doniger, director of the climate and clean air program at the Natural Resources Defense Council, referencing the EPA’s conclusion that climate-changing emissions are a threat to human health. “That would be enormously difficult, and there’s not a chance the courts will uphold that.”

The process of reversing regulations requires the same draft rulemaking, public comment and final rulemaking as issuing them — a long, plodding, bureaucratic slog.

“It’s just as difficult to tear the building down as it was to build the building up,” Doniger said. Plus, the Trump administration would invariably be sued by the same environmentalists who sued to get regulatory action on climate change in the first place.

It’s just as difficult to tear the building down as it was to build the building up. David Doniger, Natural Resources Defense Council

Passing legislation through Congress might have prevented some of the legal wrangling we see today over the Clean Power Plan. “Obviously, if it provided the contours of the policy so we wouldn’t be tied up in litigation and delays, and provided regulatory certainty and a way to raise revenues you could then plow back in things like efficiency, renewables ― yes, I would have preferred that, like many people, including the president,” said Vicki Arroyo, executive director of the Georgetown Climate Center at Georgetown Law. “But in the absence of that, I feel like how the EPA used the Clean Air Act was very responsive to the comments of the states, companies and the affected public in general.”

Obama’s regulatory approach might also get the country farther down the path toward cutting emissions. “I think most people would agree that [the Waxman-Markey bill] was economy-wide, that was important, but there were a lot of puts and takes at the end,” said Carol Browner, who served as Obama’s top climate and energy adviser in his first term, and as the EPA administrator under President Bill Clinton. “So the actual amount of reduction it would be achieving today is less than the reductions Obama is getting. Who knows when we would have started to getting reductions under [Waxman-Markey]?”

If Browner has a regret, it’s that Waxman-Markey might have tried to do too much at once. In retrospect, she said, legislation that set standards on a sector-by-sector basis might have been more feasible.

“Obviously, I want something economy-wide, since I think the problem warrants it,” she said. “But in terms of the politics of passing something, I think that would have been a little easier. But we’ll never know, right?”

Clarification: Language has been amended to better describe the present status of the legal challenges to the Obama administration’s climate rules.