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Democrats and environmentalists have made a political weapon out of presumptive Republican nominee Trump’s threat to end the Paris climate deal.

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The promise looks likely to linger in the general election, but it has a practical purpose right now, too: using Trump as a climate change boogeyman could serve to attract otherwise disillusioned Sanders supporters to her side.

“Talking about their very different view of environmental action, we see a very, very sharp contrast between the two,” Seth Stein, a spokesman for League of Conservation Voters, said of Trump and Clinton. “And we believe voters are going to see it as well.”

Clinton and Sanders waged a host of battles over environmental policies during their primary campaign, with Sanders saying Clinton is weaker than he is on issues like fracking and fossil fuel development.

Since Clinton claimed clinched the Democratic nomination this week, many Democrats, from President Obama on down, have looked to unify Sanders’ supporters with Clinton’s.

Kevin Curtis, the executive director of the NRDC Action Fund, said the spectre of what a Trump presidency would mean for the climate deal could help in that task.

“The primary is over. The conversation and the game now is how peace is made between the two principles and their supporters and how does that all come together,” he said. “Now the debate is: versus Trump, he’s going to roll back Paris. The choice is clear for anybody who cares about climate.”

Trump in May offered his sharpest position yet on the Paris deal, a landmark international accord that has countries set individual greenhouse gas reduction targets as a way to take on climate change.

“We’re going to cancel the Paris climate agreement — unbelievable — and stop all payments of the United States tax dollars to UN global warming programs,” Trump said during a speech on energy policy in North Dakota. Days earlier, he told Reuters the deal was unfair to the United States because other countries weren’t planning on cutting their emissions as quickly as the Obama administration proposed.

“This agreement gives foreign bureaucrats control over our energy and how much we use right here in America. So foreign bureaucrats are going to be controlling what we're using, and what we're doing on our land in our country,” Trump said. “No way, no way.”

Hatching the Paris climate deal was a major foreign policy victory for the Obama administration, and one both Democratic presidential candidates have said they would grow. So environmentalists naturally sounded alarms when Trump promised dropping out of the deal of the deal if he’s elected.

Officials — both American and international — have questioned whether Trump actually could end the deal. But his threat to even consider it added fuel to environmentalist concerns about his candidacy.

The Paris deal featured prominently in a string of endorsements Clinton won from green groups after Trump’s speech.

“The Republican nominee, Donald Trump, has called climate change a ‘hoax’ and said he will tear up the Paris climate agreement,” California Gov. Jerry Brown (D) wrote in an open letter endorsing Clinton.

“Hillary Clinton has the deep knowledge and diplomatic skills needed to fight for our future, preserve America’s leadership role in the Paris Climate Agreement and build upon that global framework to fight climate change,” the NRDC Action Fund wrote in its endorsement of her.

“Donald Trump, on the other hand, has recently outlined a disastrous and frankly nonsensical environmental agenda – suggesting that he would tear up the Paris climate agreement.”

“When he gets to specifics, Trump makes rash promises, like ripping up the Paris Climate Agreement, a landmark agreement that brought 196 countries together for the first time in history,” Sierra Club Executive Director Michael Brune said in the group’s endorsement of Clinton this week.

“The gap on environmental and climate issues between Secretary Clinton and Donald Trump is the largest in U.S. political history.”

For Democrats, Trump’s Paris promise serves another purpose: they are already using it a tool with which to bludgeon the Republican’s understanding of foreign policy.

Attacking Trump’s temperament and understanding of the issues has already turned into an frequent hit for Clinton. John Podesta, her campaign chairman, told a gathering of LCV members on Wednesday that, “Donald Trump is a serious candidate for president but he is not a serious man,” citing, among other things, his environmental positions and contrasting them with Clinton’s.

“Hillary’s plan will more than fulfill the United State’s commitment in Paris and keep America at the forefront of the global fight against climate change,” he said.

“We have to make sure climate denial does not find home in the White House,” he said.

Stein, whose League of Conservation Voters endorsed Clinton in the fall, said the Paris deal will remain an issue deep into the general election.

“Paris is going to be one of those major proof points,” he said. “There are a lot of points — and this is a great one in making the contrast clear — but there are a lot of points in making it clear how different they are on the environment and environment protection.”