The Democratic National Committee took a stance on climate change Thursday in San Francisco that could alienate young voters — a key group that Democrats need to be energized for the party to win back the White House.

The action that angered young environmental activists happened during the first hours of the party’s three-day conference. It wasn’t over policy — unlike many Republicans, Democrats support the science supporting the conclusion that human activity is contributing to climate change.

Instead, the argument is over how to talk about the topic. The activists wanted the Democratic National Committee to sponsor a presidential debate focused solely on climate change. That doesn’t seem like a big ask, especially not for the most existential of all issues, they say.

But the party’s resolutions committee disagreed and voted against recommending that the party support a candidates debate. Members didn’t disagree that climate change is an existential threat. But they did fear that if they sponsored a climate-focused debate, they’d have to sponsor a debate for every other constituency.

Now Playing:

One Democratic committee member, Craig Smith of Florida, recalled that there were 30 debates during the 2008 presidential campaign — so many that during one month, “the candidates spent more time doing debate prep with their staffs than they did getting to meet actual voters.”

Among those hoping the Democrats would act differently was 22-year-old Ava Mohsenin of San Francisco. She read last October’s United Nations’ report that said the planet could experience intense damage if humans don’t take large-scale action to lessen climate change by 2030.

“In 11 years, we will live in a slightly less inhabitable planet. That is terrifying to me,” said Mohsenin, one of the 200 activists from the Sunrise Movement environmental group who were at the Democrats’ meeting Thursday urging the party to sponsor a debate.

Ten Democratic presidential candidates have agreed to take part in a CNN town hall forum on climate issues Sept. 4. But activists say that’s not as good as a debate, where the contenders can challenge each other’s positions in back-and-forth exchanges. Forums also typically draw fewer viewers than party-sponsored debates.

“Those interactions between the candidates are going to help us weed out who the best candidate on climate is,” Mohsenin said.

Many of her peers are as passionate about climate change as Mohsenin is.

NextGen America, a political group that organizes young people nationally, collected more than 300,000 surveys from newly registered voters in 11 swing states last year. “Climate change always ranks as a top issue, along with the cost of college,” said communications director Olivia Bercow. “The DNC is doing a disservice to young voters” if it doesn’t hold a climate-change debate.

Environmental activists warn that the party could also be doing a disservice to itself: In 2020, Millennials and their younger counterparts in Gen Z are projected to make up 37% of the electorate, according to Pew Research.

“The 18- to 35-year-old vote is what we absolutely need as a party. And this issue is far, far, far more important to that age demographic than anything else,” said Jessica Chambers, 38, a party member from Wyoming who supports the young activists’ position. “They’re staring their future in the face.”

Party leaders respond that it’s nothing personal about climate change. It’s political. And they haven’t been swayed by debate supporters presenting them with a petition signed by 226,000 people, or by a YouGov.com online survey last month that found 64% of registered Democrats supported such a debate.

Party leaders say it would invite pressure from people who feel passionately about other causes. Just this week, AARP asked party chair Tom Perez to hold a debate on issues affecting senior citizens. End Citizens United, a group that hopes to lessen the influence of corporate campaign cash, wants the party to hold a debate on that topic. The gun control group Giffords wants one on dealing with firearms violence.

Perez says he’s sympathetic to climate-change activists. He wrote in a Medium post in June that “beginning in 2017, I made clear to our media partners that the issue of climate change must be featured prominently in our debates. That didn’t happen in 2016 — and it was wrong.”

But that doesn’t mean climate should get its own debate, he says. Perez fears that could be seen as the party favoring a candidate such as Tom Steyer, the former hedge fund manager, who has made dealing with climate change a central theme of his campaign.

“How do we say no to other candidates in the race who may request debates focused on an issue they’ve made central to their own campaigns?” Perez wrote.

On Wednesday, veteran Democratic media strategist Tad Devine — who advised the presidential campaigns of Al Gore, John Kerry and Bernie Sanders’ 2016 effort — tweeted that “to amend these rules now, after having enforced them throughout this primary process, would be putting our thumb on the scale.”

For the DNC to sanction single issue debates on climate or other important topics would be like opening up Pandora’s box. The fair process that have put in place ain’t broken so they shouldn’t try to fix it. — Tad Devine (@taddevine) August 21, 2019

That’s what the party is really afraid of here. Sanders’ supporters, in particular, are convinced party leaders had both thumbs on the scale for Hillary Clinton in 2016, and Perez says he’s determined to avoid a repeat.

“We learned a valuable lesson in 2016 that, in order to ensure the strength of our party and the trust of our voters, the DNC must remain neutral in both practice and perception,” Perez wrote in his Medium post.

The flip side is how people like Mohsenin will react. In the near-term, they will press the full party to reconsider the idea Saturday when members will vote on resolutions.

But many activists will continue their fight. This weekend in Berkeley, the Sunrise Movement will host a training session for 600 climate activists to prepare for 2020 campaigns and beyond. Mohsenin says they’ll take note of how strongly the Democratic Party gets behind their cause.

“We are young right now, but in a very short time, we will be the core of the Democratic Party,” Mohsenin said. “We are not the radical left of the Democratic Party. We’re just the mainstream of our generation.”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli