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Democratic presidential front-runner Hillary Rodham Clinton pledged Sunday that if elected she will build on a new White House clean-energy program and defend it against those she called "Republican doubters and defeatists."

Clinton was the first 2016 candidate to respond to the ambitious plan that President Obama will debut on Monday. Details of the program, which aims to cut greenhouse-gas pollution, were released over the weekend. The new regulation will require every state to reduce emissions from coal-burning power plants.

In a statement Sunday, Clinton called the plan "a significant step forward in meeting the urgent threat of climate change."

Environmental issues are playing a larger role in the 2016 election than in any recent cycle and are an increasingly important marker for Democratic candidates.

Former Maryland governor Martin O'Malley, who is also seeking the Democratic nomination but lags far behind Clinton in the polls, praised the Obama plan in a Twitter message.

He also added a link to his own climate plan.  

Sen. Bernie Sanders (I-Vt.), who is the only Democratic candidate besides Clinton polling in double digits, said Sunday in a statement, “President Obama understands that climate change is the great planetary crisis facing us and that we must move boldly to transform or energy system away from fossil fuels and toward energy efficiency and renewable energy."

He added, “I have not yet seen all of the details of the president’s proposal but it sounds to me like a step forward in ending our dependence on fossil fuel and I support that effort.”

Sanders picked up an endorsement Saturday from a national environmental group. The political arm of Friends of the Earth, which bills itself as the world's largest grass-roots environmental network, was the first national group to formally endorse Sanders's bid.

Sanders has also won points among progressives for opposing the Keystone XL pipeline, which is widely unpopular on the left, while chiding Clinton for refusing to say whether she backs the proposed Canada-to-U.S. oil line. Clinton says she cannot weigh in as a candidate, because she had a direct hand in beginning the pipeline review process while she was Obama's first-term secretary of state.

In his statement, Sanders reiterated his opposition to the pipeline project. “We must also kill the Keystone XL pipeline which would facilitate the excavation and transportation of some of the dirtiest fuel on the planet," he said.

Speaking of the new Obama rule, Clinton said, "It sets a smart federal standard that gives states the flexibility to choose how to reduce carbon pollution most effectively."

She added: "It drives investments in clean energy and energy efficiency, reduces asthma attacks and premature deaths, and promotes a healthier environment and a stronger economy.  It’s a good plan, and as president, I’d defend it."

The retooled version of the administration’s Clean Power Plan, first proposed a year ago, would speed up the shift to renewable energy while setting tougher goals for slashing carbon emissions blamed for global warming, The Washington Post reported Sunday, citing administration officials briefed on the details.

The rule would regulate carbon emissions as a pollutant and is certain to face legal challenges, as well as fierce opposition from the Republican-controlled Congress. Opponents blasted last year’s proposed regulation, calling it a possibly illegal federal overreach that would impose costly burdens on utility companies and their customers.

“It will need defending," Clinton said. "Because Republican doubters and defeatists — including every Republican candidate for president — won’t offer any credible solution. The truth is, they don’t want one."

Many among the large Republican field have criticized Democratic climate proposals in the past, including the one Clinton unveiled last month in Iowa, and are likely to weigh in on Obama's program. Clinton appeared to anticipate those responses in her statement Sunday.

"They just keep making the same tired arguments they’ve been making for years," she said. "They refuse to accept science."

Republican National Committee Chairman Reince Priebus tied Clinton's support for what he called Obama's "job-killing EPA regulations" to her economic agenda.

“President Obama’s heavy-handed EPA regulations won’t impact the climate, but they will have devastating consequences for our economy," Priebus said in a statement Sunday. "The last three months saw the slowest wage growth in 33 years, and now President Obama and Hillary Clinton want to make it even harder for struggling Americans to make ends meet. . . . It's becoming clearer by the day that Hillary Clinton doesn’t have answers to get our economy growing again.”

Former Florida governor Jeb Bush, who is seeking the Republican presidential nomination, called the Obama plan "irresponsible and overreaching."

"The rule runs over state governments, will throw countless people out of work, and increases everyone’s energy prices," he said in a statement issued by his campaign.

"Climate change will not be solved by grabbing power from states or slowly hollowing out our economy," Bush added. "The real challenge is how do we grow and prosper in order to foster more game-changing innovations and give us the resources we need to solve problems like this one.”

The new plan sets a goal of cutting carbon pollution from power plants by 32 percent by 2030, compared with 2005 levels. It would reward states and utility companies that move quickly to expand their investment in solar and wind power.

Clinton called the plan "the floor, not the ceiling," and said she would go further.

Her plan contemplates using executive orders if Congress tried to block her proposals.

The Clinton program was praised by some activists, including clean energy evangelist Tom Steyer, but others have said that it doesn't go far enough. It calls for the installation of more than 500 million solar panels across the country within her first term and powering every home in the United States with renewable energy within 10 years.

In a video presentation of her plan, Clinton took swipes at several Republican presidential candidates, displaying some of their past comments questioning whether climate change is real. One attributed to Bush read: "I'm a skeptic. I'm not a scientist." Another, attributed to Rand Paul: "It's absolutely and utterly untrue."

Also Sunday, industry and environmental groups weighed in on the Obama plan.

Major coal mining and utility industry groups condemned the final Clean Power Plan details that were reported over the weekend, but some individual companies are already well on the way toward meeting these targets.

Scott Segal, director of the Electric Reliability Coordinating Council, a group of power companies subject to the rule, said the new plan got worse during the drafting process.

"They have made a proposed plan which strains the power system even more restrictive, calling for reliable and cost effective energy sources to be replaced by intermittent, costly sources largely incapable of meeting base load power needs in the U.S.," Segal said in a statement.

The National Mining Association representing the coal industry took a dim view of the widely reported rules the EPA finalized after making some changes from its earlier version. And it said that the EPA, which is leaving flexibility for states to come up with strategies to meet targets, was giving the nation’s governors an unappealing choice.

“These vaunted ‘changes’ matched with tougher standards are like a teaser rate on a costly loan,” said Luke Popovich, a spokesman for the NMA. “The calculation for governors is the change: reject this deal, or accept it and stick consumers with the bill for the president’s ‘legacy.’”

But the chief executive of AEP, the utility that is one of the largest carbon emitters in the country, said that the EPA’s decision to delay implementation of some emissions cutbacks by two years was “positive” and would leave more room for technology developments to ease the path toward the agency’s targets.

“The utility industry is already moving in this direction,” said Nicholas Akins, AEP’s chief executive.

“This is huge in terms of the tentacles it has throughout the industry,” Akins said. “I think the verdict is still out on some key points, but I think it’s positive that they recognize that a delay is necessary on the implementation to allow the states to really take a look at what they need to do, and good that they recognize that we don’t know what the transition path is.” By delaying implementation, he added, “technology can catch up.”

AEP has already trimmed its carbon dioxide emissions by 15 percent compared with 2005, mostly by retiring a quarter of its coal-fired power plant fleet and adding natural gas and renewables. It is already planning to cut coal from 61 percent of its current fuel for electricity to 51 percent by next year and 48 percent by 2026 and boost renewables (including hydropower, wind, solar and pumped storage) to 15 percent from the current level of 8 percent.

Asked whether he supported Senate Majority Leader Mitch McConnell’s call for governors to ignore the EPA’s call for state plans, Akins said that if the plan “on face value doesn’t pass muster, they have to stand up and say something. But if it’s anything possible, we got to be working with it.”

He said that he expected technology advances to make utility scale solar more economic, but he said the EPA shouldn’t force early retirement of fossil fuels, coal or natural gas. “I’m a little troubled by the notion that we want to get rid of fossil fuels and move heavily toward energy efficiency and the renewable side of things,” Akins said

Fred Krupp, president of the Environmental Defense Fund, praised the climate change plan for addressing what he called the "biggest threat facing us right now."

“With this historic announcement, the United States is making clear that it is no longer acceptable to put unlimited amounts of climate pollution into our air," Krupp said. "The move toward a world safe from climate change is beginning in earnest."

Joby Warrick and Steven Mufson contributed to this report.