President Barack Obama waves to the press after stepping off Marine One helicopter on the South Lawn as he returns from a weekend at Camp David to the White House, August 2, 2015, in Washington, DC.National Journal

Riding in his fortress of a Cadillac that gets just 3.7 miles to the gallon, President Obama knew it was time to go big on climate change.

After his reelection in 2012, Obama rode in "The Beast" with top climate aide Heather Zichal coming back from a meeting at the Energy Department. They were talking casually about an environmental strategy for his second term, when she found herself promoting the idea that Obama "throw everything he can" into a climate plan to guide his next four years.

"His immediate reaction wasn't, 'Well, what do you think the politics of that are, or wouldn't it be hard to do?' There was no question," Zichal recalled. "It was just, 'That's a great idea. But we have to go big, and we have to go bold.'"

Monday's official announcement of sweeping Environmental Protection Agency rules that impose a first-ever mandate to cut carbon pollution from the nation's coal-fired power plants, by far the largest source of unchecked emissions, is the most important result.

This is second-term Obama. A president who routinely reminds people that he never has to run for office again using the power of the executive branch to sidestep an opposition Congress to accomplish legacy-defining agenda items. If Congress won't act on energy and immigration, the White House will.

But despite going far beyond his predecessors on global warming, Obama's green legacy will face question marks for years. Executive actions such as new EPA regulations lack the certainty of the bedrock environmental statutes Congress passed in the late 1960s and early 1970s.

The particulars of how he's using the Clean Air Act to demand cuts in power-plant pollution are almost certain to wind up before the Supreme Court. And GOP White House contenders, if elected, would seek to dismantle or hamstring the rules, which have already emerged as a top target of congressional Republicans.

Obama's green legacy is not yet set. He has yet to announce a decision on whether or not to approve the Keystone XL pipeline, which environmentalists bitterly oppose, believing Keystone will enable a surge in carbon-spewing oil-sands production in Canada. And greens are still unhappy he is allowing Shell to drill for oil off the Alaskan coast in the Arctic Ocean.

And another legacy item awaits: The United States is heavily involved in negotiations aimed at reaching a major global climate accord at a make-or-break United Nations summit in Paris late this year.

"He really sees this as just an issue that just can't be ducked," said Dan Utech, a White House adviser on energy and climate. "For both today, but also for the sake of his daughters, their kids, and the kids and grandkids of all Americans."

Obama has talked about how, as an undergraduate in Los Angeles, the pollution was so bad that "folks couldn't go outside." And he has blamed rising temperatures for his daughter Malia's asthma attacks when she was 4.

Of course, the sweeping EPA rules weren't the first choice for the administration. An extensive climate-change bill to set up a national cap-and-trade system passed the House in mid-2009 but collapsed in the Senate the next year.

Presidents are often remembered for sweeping actions and bills they signed, like the civil rights and anti-poverty bills that President Lyndon Johnson pushed through Congress. Carol Browner, Obama's former climate-policy director, said Obama doesn't get the credit he deserves for his step-by-step approach on climate change.

"People don't see the arc of it. They see each of these acts as isolated. But when you put them together, they're actually more than the sum of the parts," Browner said. "It all comes together to give you a magnitude of reduction."

The stimulus Obama ultimately signed in 2009 would steer some $90 billion into low-carbon energy initiatives and technologies. Another big win on climate in his first term: rules finalized in 2012 that boost gas mileage standards for cars and light trucks to 54.5 miles per gallon by 2025, a mandate that will make the United States auto fleet much greener.

William Reilly, who led EPA under President George H.W. Bush, strongly praised Obama for his steps to address global warming, especially in the face of strong opposition from congressional Republicans. Reilly credits Obama for tackling a topic that had been "suspended as a presidential priority for the previous eight years" under President George W. Bush.

Reilly cited the big boost in auto mileage regulations as an example of an accomplishment that has received too little praise. "People have paid too little attention to the consequences of that," he said.

Obama's GOP foes — including the ones hoping to succeed him — say the rules will leave a legacy of a different sort: one of a president willing to impose overaggressive green rules that hurt the nation's economic competitiveness. "The rule runs over state governments, will throw countless people out of work, and increases everyone's energy prices," Jeb Bush, one of the leading GOP White House contenders, said Sunday.

The criticism from Bush and other Republicans underscores a risk for Obama: The problem with presidential directives and international diplomacy, is they can be undone by whoever succeeds him.

"The scary thing is that despite all of the great things that the president has put in motion, it is very easy for a Republican to unravel those," Zichal said. "In the same way President Obama had the existing authority to have such a tremendous regulatory agenda, a [new] president could easily have an agenda to unwind that."

In the nearer term, the White House is promising to fight hard to keep the EPA rules intact. McDonough last week vowed to "veto ideological riders" from Capitol Hill aimed at thwarting the rules. "We will not back down," he said.

Internationally, Obama struck a landmark climate deal with China late last year. China agreed to have its rising carbon emissions peak by 2030 at the latest. Obama pledged domestic reductions of 26 to 28 percent below 2005 levels by 2025. (U.S. carbon emissions are already down about 10 percent over the past decade.) White House officials hope the accord between the world's top two greenhouse-gas polluters will energize the effort to strike a global pact in Paris.

If Obama's muscular executive agenda isn't dismantled by a future president or Congress, Princeton University political historian Julian Zelizer said Obama has earned a spot on a Mount Rushmore of environmental presidents alongside people like Teddy Roosevelt, Lyndon Johnson, and Richard Nixon (who established the EPA).

"He deserves a spot in that he has used executive power to deal with this, he has now talked about it, he has come back to a theme that he kind of left behind in 2008," Zelizer said.

Zelizer believes, however, that Obama's heavy focus on climate is too late to make it among the things he's most remembered for. "He made a decision in terms of dealing with what should come first and where he was going to burn his political capital — on health care, on stimulus," he said. "Part of that was pragmatic, part of it was a sense of crisis, but he didn't really tackle climate change until now, and it's late in his presidency. In some ways, in terms of shaping what he's about and the themes that he will be remembered for, I think it is other issues."

But Obama, his allies say, has always been engaged on green energy and climate change from the beginning.

Just a month after his election, on December 16, 2008, the soon-to-be president met with his transition team in Chicago to talk about how to handle the fiscal crisis. Discussing what would be included in the massive economic stimulus, he told his advisers, including Browner, "I want a big commitment to green energy. A big investment."

"He's been consistent from the beginning of his presidency that he would work on this, that he could use the tools available to him," Browner said. "Climate change is a complex issue. There's not a silver bullet."

On the policy debates, he leaves the particulars up to trusted staffers.

"He is definitely engaged in the substance decisions, but he's not going to argue in the weeds on a very detail specific component of the 111(d) regulation," Zichal said, referring to the new EPA power-plant regulations. "He's absolutely committed to making sure that all these regulatory actions line up with his broader vision and goals. And then he largely empowers the people around him that he's put so much trust in and known for years to help deliver the goals."

It's a starkly different approach than the heavily bureaucratic process Browner faced as EPA head in the Clinton administration. If she wanted to regulate ozone, she recalls, "I'd have a big debate about whether I should even regulate it. I'd have to argue about the science; I'd argue about the law. And then they'd say, 'OK, yes, you should regulate it.' Then I'd have a second debate maybe a year later, about how much I should regulate it."

With Obama, she said, administrators have much more freedom to get things done. "He puts his finger on the scale at the beginning and says: 'This is important. I want you to do this.'"

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