Donald Trump’s rejection of climate science has Hillary Clinton’s campaign – and her environmentalist allies – sensing a fresh opportunity to reach millennial voters.

The Democratic nominee highlighted her divide with Trump over climate change on the University of New Hampshire’s campus Wednesday – telling the crowd she “never thought when I gave my acceptance speech at the Democratic National Convention that I would have to put in the following sentence: I believe in science.”

“Climate change is real, it is serious and we have to be united and committed in addressing it,” she said.

Her comments come as Clinton and her allies – particularly NextGen Climate and the League of Conservation Voters – seize on a key moment in Monday night’s presidential debate.

Clinton accused Trump Monday of believing that climate change is “a hoax perpetrated by the Chinese.” Trump interjected, insisting, “I don’t say that.” However, as social media users quickly discovered, he had said exactly that in a 2012 tweet.

The four-year-old tweet went viral, becoming the most-retweeted item on Twitter during the debate – and making for a quick and easy fact-check against Trump.

The next morning, Trump’s campaign manager said Trump doesn’t believe what nearly all climate scientists consider a fact: that human activity is contributing to the warming of the earth.

“He believes that global warming is naturally occurring,” Kellyanne Conway told CNN’s Alisyn Camerota on CNN’s “New Day.” “There are shifts naturally occurring.”

When Camerota followed up to ask specifically if that means that Trump believes that climate change is not man-made, Conway responded, “Correct.”

It’s not a new position for Trump – but Clinton’s attack elevated the issue in front of a TV audience of more than 81 million people.

After the debate exchange, Clinton’s campaign, said an aide who declined to publicly discuss strategy, sees an opening on climate change – an issue that polls suggest could help the former secretary of state address a key problem area: Millennials.

Younger voters overwhelmingly backed President Barack Obama in 2012 – giving him a 29-point lead over Republican nominee Mitt Romney, according to exit polls. In 2008, Obama easily outpolled Republican John McCain among the group, 66% to 32%. But the enthusiasm that young voters – including many first-time voters – showered on Obama, however, has not transferred over to Clinton.

A national Bloomberg poll conducted by respected Iowa-based pollster Ann Selzer found Clinton has just 40% support among voters under age 35 – just a 4-point lead over Trump in a demographic key to her campaign, with Libertarian Gary Johnson peeling off 11%.

Young voters have consistently ranked climate change as an important issue, and Clinton’s position is a major divide with both Trump and Johnson, who in 2012 said that “government should not get involved in this.”

Clinton’s environmental allies are urging her campaign – as well as debate moderators – to keep climate change in the spotlight.

“We’re likely going to continue to see her drive this issue, particularly to younger voters – and voters who may be considering a third-party candidate, whether it’s Johnson or (the Green Party’s Jill) Stein,” said Clay Schroers, the League of Conservation Voters’ national campaigns director.

“I think that making it clear that she has plans to tackle this and to continue the incredible progress the Obama administration has made is really a key part of demonstrating to those voters that she’s tackling the issues that matter to them,” he said.

Helping Clinton is NextGen Climate, the group funded by liberal billionaire Tom Steyer, which has 550 field organizers on more than 200 campuses across eight swing states – all highlighting the divide between Trump and Clinton on climate change.

“Now that this has come up in such a prominent way, that’s what the next stage of the campaign should focus on,” said Jamison Foser, a senior adviser for NextGen Climate.

“This is exactly what we’ve been laying the groundwork for for a long time,” Foser said. Of Trump’s comments, he said, “It just makes the conversation easier.”

A Trump spokeswoman didn’t return CNN’s request for comment.

A Clinton spokesman, meanwhile, said she’ll continue to highlight the issue.

“From now through Election Day, Hillary Clinton will continue to lay out the choice facing millennials between electing a president who has a plan to fight climate chance and make the United States the clean energy superpower of the 21st century starting on day one, or one who says it’s a Chinese hoax and has a track record of opposing renewable energy,” said Clinton spokesman Tyrone Gayle.
“Despite overwhelming scientific evidence, Donald Trump and his campaign continue to bury their heads in the sand and peddle repeatedly debunked claims.”

One key driver of the climate change message could be Clinton’s former Democratic primary opponent.

Bernie Sanders, who easily bested Clinton among 18-29-year-old voters while making climate change a key issue in his campaign, is set to begin a more aggressive campaign schedule for Clinton – planning to hit Wisconsin, Iowa, Michigan, Ohio and Pennsylvania, as well as other states with large voting populations like California in a move designed to help Clinton win the popular vote.

Sanders appeared alongside Clinton in New Hampshire on Wednesday, urging his supporters to back her.

“I am asking you here not only to vote for Secretary Clinton, but to work hard, to get your uncles and your aunts, to get your friends, to vote,” Sanders said. “If anybody tells you that this election is not important, you ask them why the Koch brothers and Sheldon Adelson and other billionaires are spending hundreds of millions of dollars to elect their candidates.”