President Obama’s plan to reduce greenhouse-gas emissions is aimed at three major constituencies. First, there’s the plan’s immediate goal: significant decreases in the emissions in the U.S. between now and 2030. Second, the rule arrives as the world gears up for global emissions talks in Paris in December, and American action is seen as necessary to convince other countries to act. And third, Obama views the fight against climate change as an essential part of his legacy, alongside the Affordable Care Act.

“We’re the first generation to feel the impact of climate change, and the last generation that can do something about it,” Obama said at a press conference at the White House on Monday, repeating a line he’s used before. The president emphasized the moral case for reducing emissions throughout the speech, invoking Pope Francis’s call for action, and scolding “cynical” critics who charged his plan would hurt minorities and the poor. “If you care about low-income minority communities, start protecting the air they breathe and stop trying to rob them of their health care.”

In brief, the Clean Power Plan puts limits on carbon pollution from power plants, mandating a 32-percent reduction by 2030, though based on 2005 levels, setting state-by-state standards for reduction. The rule is expected to lead to the closure of many coal-fired plants and prevent new ones from opening. The regulation was first proposed last year, and, after the EPA considered public comments, the final rule was released Monday. Experts described the rule as historic.

“They’re the most important regulations on climate change ever issued by the U.S.,” said Michael Gerrard, director of the Sabin Center for Climate Change Law at Columbia Law School.

Gerrard said while the rule’s impact would be important stateside, it was at least as important because of the role it will play in the global negotiations in Paris. Although China is now the world’s largest emitter of greenhouse gases, the U.S. has historically been larger, and American action is seen as essential to urging other countries to move on reductions. Following Obama’s proposal in 2014, China announced new emissions targets. But international progress is fragile, and attempts at marshaling a global response to climate change have repeatedly foundered.

“If these rules were to crash and burn before the Paris conference, that would likely have disastrous effects there,” Gerrard said.

The rule faces serious challenges in the U.S., too. A coalition of Republican politicians and energy corporations have lined up to oppose it. Senate Majority Leader Mitch McConnell, whose home state of Kentucky is a major coal producer, has urged state governments not to comply. Even before the rule was finalized, challenges from coal companies and the state of West Virginia tried to stop it, but the D.C. Circuit Court ruled that a rule that hadn’t been enacted couldn’t be challenged. Those plaintiffs will likely return to the courts for a second chance.

The final rule is similar to the proposal—and in fact upgraded the target reduction from 30 percent to 32 percent. But it also makes several changes aimed at making the rule more palatable to states and more likely to win approval from judges.

“Some of those changes were made to make it more—even more, I would say—legally defensible,” said Vicki Arroyo, executive director of the Georgetown Climate Center at Georgetown University. Those changes seek to offer states more flexibility in reaching targets, particularly early in the process. They’ll have more time to comply, be able to participate in an emission-credit trading market, and receive incentives to begin the process. States’ specific targets have been recalculated in the final rule.

Will these adjustments bring any reluctant states around? “I think they will," Arroyo said. “I think we might lose a couple states that have been pretty constructive before. Some people will react favorably to their new numbers, and some might gulp and try to figure out what it means to them.”

David Thornton, assistant commissioner of the Minnesota Pollution Control Agency, was one state regulator who was initially pleased with the new rule.

“This final version is a better, improved plan than the one they came out with over a year ago,” he said. “We were concerned that the original plan was not well balanced in terms of what it required individual states to do. We think this new version is more fair, especially more fair to states that have already taken action to reduce their emissions.”

In states that haven’t been as proactive, however, the new rule may not win new supporters. As Coral Davenport notes, Republican attorneys general from around the nation are meeting in West Virginia, and they’re expected to challenge the law in the courts. The White House hopes that whatever changes don’t win over state governments will at least convince judges to uphold the rule—and the international community and posterity to look kindly on it as well.