And Sen. John Neely Kennedy, who is expected to announce Monday whether he will run for Louisiana governor, told reporters last week that while the Earth may be getting hotter, “I’ve seen many persuasive arguments that it’s just a continuation of the warming up from the Little Ice Age.”

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As President Trump’s rejection of climate science isolates the United States on the world stage, illustrated by the small U.S. delegation dispatched to this week’s United Nations climate summit in Poland, he has also presided over a transformation in the Republican Party — placing climate change skepticism squarely in the GOP’s ideological mainstream.

Where the last Republican president, George W. Bush, acknowledged that the Earth was warming and that “an increase in greenhouse gases caused by humans is contributing to the problem,” the prevailing GOP view expressed on the campaign trail this year and espoused by many members of Congress is built on the false premise that climate science is an open question.

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The small number of voices supporting the science have been largely drowned out.

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The House Climate Solutions Caucus, co-founded in 2016 by Rep. Carlos Curbelo (R-Fla.) and once thought of as a catalyst for climate-friendly legislation, lost 24 of its 45 Republican members to retirement or election defeat this year — including Curbelo. An analysis by the liberal Center for American Progress Action Fund found that 61 percent of Republicans in Congress have in some way raised doubts about climate change, deflected the question, claimed that the climate is always changing, or questioned the extent to which humans contribute to climate change.

As a result, the two major political parties, which not long ago largely shared a fundamental view on the challenges posed by climate change, are now battling over the credibility of science and facts — a fight that is shaping up as a potentially defining issue of the 2020 presidential campaign.

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“There are some things that started to emerge as a consensus, which is Republicans had said the climate is changing, and we’re going to have to do something about it,” said Douglas Holtz-Eakin, who was a top economic adviser to Bush and 2008 Republican presidential nominee John McCain. “The president’s stance makes that harder because he denies it’s even changing. . . . This puts us back into gridlock again.”

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The skeptics’ impact on U.S. policy has been laid bare in recent days. Trump shrugged off his administration’s 1,600-page report outlining the severe threats of climate change. Then, over the weekend, his team secured language in a joint statement issued by Group of 20 leaders over the weekend carving out a separate U.S. position on climate goals and reaffirming the U.S. withdrawal from the Paris climate accord.

That Earth’s temperature is rising is not in dispute among most scientists or other world leaders. A 2013 report that analyzed scientific papers studying climate change found that, of those papers that took a position on the matter, 97 percent endorsed the idea that humans are causing global warming.

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The planet has warmed nearly 2 degrees Fahrenheit above late 19th-century levels, and scientists say that unless carbon emissions fall sharply in the coming years, the worst consequences of climate change will be unavoidable.

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Pope Francis called for a “revolution” to combat climate change, saying the consensus was clear and “doomsday predictions can no longer be met with irony or disdain.”

But after the U.S. government released its long-awaited climate assessment on Nov. 23, most Republicans followed Trump’s lead in downplaying the catastrophic warnings of more wildfires and flooding, damage to human health, and a substantial hit to the U.S. economy.

“Our climate always changes, and we see those ebb and flows through time,” Sen. Joni Ernst (R-Iowa) said on CNN’s “State of the Union.”

Sen. Ben Sasse (R-Neb.) said that while the report is important, there is too much “alarmism” around climate science.

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“I think the real question, though, becomes what do you do about it? Because you can’t legislate or regulate your way into the past,” he said on “Fox News Sunday.” “Right now, you don’t hear a lot of people who put climate as a No. 1 issue, you don’t hear a lot of them offering constructive, innovative solutions for the future — it’s usually just a lot of alarmism.”

Polling suggests there has been movement among the electorate. Nearly two-thirds of Republicans believe the world’s climate is changing, according to a recent Monmouth University survey, a jump from 49 percent three years ago.

Among Democrats, 92 percent say that climate change is occurring — and 82 percent say they consider it “very serious.” Among independents, 78 percent believe in it.

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“President Trump is becoming a real caricature of climate disputation,” said Bob Inglis, a former Republican congressman from South Carolina who now runs a group called republicEn that tries to convince conservatives to address climate change. “The public is coming to much greater awareness and understanding of the challenge, and he persists in this basically superstitious denial of the data.”

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Inglis said he saw some positive signs in recent days — such as Sen. Thom Tillis (R-N.C.), who once denied climate change, calling on both parties to combat it — but also reason for pessimism.

“There’s still disappointments along the way,” Inglis said. “But I think that’s the way it is any time you’re trying to do something very big. Like in civil rights, it was two steps forward, one step back. That’s what we’re dealing with here.”

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Several of those questioning the science — including Blackburn and Kennedy — did not respond to requests for comment.

Asked about Scott’s reticence to talk about climate change — he often says, “I’m not a scientist,” and his campaign website’s section on the environment never mentions climate change — a spokesman pointed toward some of his efforts as Florida’s governor.

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Scott has supported increased funding for the state Department of Environmental Protection and implemented a $3.6 million program aimed at preparing coastal communities “for the effects of climate change, especially rising sea levels.”

Asked in October whether Hurricane Michael demonstrated the effects of climate change, he appeared to acknowledge that sea temperatures were rising but dismissed the cause as inexplicable.

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“Here’s what we know: This storm came up really fast. And the waters were hot,” he said. “Do we all know why that happened? None of us know why that happened.”

Danielle Pletka, a foreign policy expert at the conservative American Enterprise Institute, was criticized last week after an appearance on NBC’s “Meet the Press” during which she questioned climate change and said, “We shouldn’t be hysterical.”

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In an interview, she said she was taken aback by the criticism, some of which said she was wrong to claim that the world just experienced two of the coldest years since the 1980s. PolitiFact, noting that they were actually among the warmest, rated her claim false.

“I never realized this is an issue people are mentally incompetent, verging on — hysteria is not the right word, it’s worse than that,” Pletka said. “I don’t know if it’s all wrapped up with the antipathy toward Trump and the Trump derangement that has afflicted so many people.”

Sen. Lindsey O. Graham (R-S.C.), who has become a staunch Trump ally since running against him for president in 2016, has consistently attempted to push his party to address climate change. He worked in 2010 with then-Sen. John F. Kerry (D-Mass.) to try to pass comprehensive climate and energy legislation, an effort that ultimately failed. Graham has endorsed a carbon tax and challenged his party to stop giving credence to the out­liers who question the science.

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“Here’s a question you need to ask everybody running as a Republican: What is the environmental policy of the Republican Party?” Graham said on CNN during his 2016 presidential campaign. “When I ask that question, I get a blank stare.”

Graham did not comment on Trump’s response to the recent climate assessment, and he declined a request for an interview on the topic. His spokesman said that his views have not changed.

Sen.-elect Mitt Romney (R-Utah) also has sounded alarms within his party, saying that he believes climate change is happening and that humans are a factor. During his Senate campaign in August, he said the changing climate will make wildfires more common and more dangerous in Utah.

During an address before college students in St. Louis last year, Romney said he was “concerned about the anti-scientific attitude” from members of his party.

“I happen to believe that there is climate change, and I think humans contribute to it in a substantial way, and therefore I look with openness to all the ideas that might be able to address that,” he said. “The idea of doing nothing, in my view, is a recipe for disaster . . . it’s going to require presidential leadership.”