Democratic leadership again considers climate change debate -- and again says no Democrats met in San Francisco for three days of meetings with party insiders.

Democrats have shut the door to a presidential debate focused on climate change.

The proposal dominated the party's convention this week in San Francisco and pitted party officials who oppose single-issue debates against activists, who see climate change as an existential threat that deserves special attention heading into the 2020 election.

On Thursday, the proposal failed in the Democratic National Committee's resolutions committee, and on Saturday the DNC leadership delivered a final "no" vote.

DNC Chair Tom Perez, who opposed the proposal, said the committee has received "dozens" of requests for single-issue debates, all on "compelling issues," and that it would be a mistake to "change the rules in the middle of the process."

He also pointed out that the party has allowed candidates to appear at single-issue forums and town halls, and that many candidates will participate in a series of climate change forums on CNN and MSNBC this fall.

But a number of candidates, including Massachusetts Senator Elizabeth Warren and former Texas Congressman Beto O'Rourke, have endorsed the idea.

"This decision is as baffling as it is alarming. Our planet is burning— the least we can do as a party is debate what to do about it," O'Rourke said on Twitter following the vote.

Washington Gov. Jay Inslee, who made climate change the central issue of his campaign and proposed the idea of the debate in the first place, dropped out of the race this week.

Meanwhile, activists who argued that the singular threat posed by climate change set it apart from other issues denounced the DNC's decision.

“This is downright irresponsible. Climate change is an emergency, but Tom Perez isn’t acting like it,” said Sofie Karasek, a spokesperson with Sunrise Movement, a youth-led environmental group. “We have just over ten years to completely transform our economy to avert catastrophe, but instead of being the adult in the room, Tom Perez is throwing procedural temper tantrums.”

Proponents argued that a climate debate would help elevate the issue and force the eventual Democratic nominee and lawmakers to take the issue seriously.

Young people held more than 20 rallies and sit-ins at Democratic Party offices in support of the proposal in recent weeks, garnered the endorsements of 24 state party chairs and over 100 voting members, and the backing of dozens of city and county Democratic parties. 64% of Democratic voters support it, and a coalition of groups supporting the resolution collected over 540,000 signatures.

“The Democratic Party needs the energy and motivation of young people to win in 2020. The energy around this issue has been incredibly clear, yet Tom Perez keeps shooting the party in the foot by rejecting that energy and turning it away,” said Evan Weber, Sunrise Movement political director. “Without hundreds of thousands of people raising their voices, we never would have gotten the town halls on and CNN and MSNBC. This is the kind of energy we need from young people to win in 2020.”

Stifling debate?

In June, the DNC saiud it would block candidates from participating in third-party debates, and refused to host one on climate change themselves.

"If we change our guidelines at the request of one candidate who has made climate change their campaign’s signature issue, how do we say no to the numerous other requests we’ve had?" Perez wrote in a blog post at the time. "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?"

Tina Podlodowski, the main sponsor of the original climate debate resolution and the chair of the Washington State Democratic Party pointed out that the DNC leadership had changed the debate rules more than once already.

"Look no further than the escalating requirements to qualify for the debates themselves,” she said. "We are not asking for a single issue debate -- we are asking the DNC to recognize the urgency of this crisis and how it touches every issue that motivates the young people and union workers we must turn out to win in 2020.”

Indeed, the party's criteria for how candidates qualify for the debates was another major sticking point over three days of meetings at the Hilton San Francisco Union Square hotel.

Twelve presidential candidates addressed party members and insiders on Friday, while Rep. Seth Moulton, D-Massachusetts, used his speaking slot to drop out of the presidential race.

Sen. Michael Bennet, D-Colorado, took aim at the party’s rules for debate qualification ahead of the third primary debate in Houston, which Bennet has not yet met.

“The DNC’s process is stifling debate at a time when we need it most,” he said, to some of the loudest cheers of the morning from party members. “We’re rewarding celebrity candidates with millions of Twitter followers, billionaires who buy their way onto the debate stage, and candidates who have been running for president for years.”

The DNC required that candidates have at least 130,000 individual donors or reach two percent in four national or early state polls certified by the party.

While the summer meeting represented a unique opportunity for candidates to glad-hand party officials from across the country, the conference is not the same power center it once was.

After Sanders and his supporters accused party insiders of undermining his support in the primaries in 2016, the party diluted the influence of the party’s superdelegates in the nominating process, preventing them from voting in the first round of ballots at next year's convention in Milwaukee.

With the influence of party insiders diminished, some of the candidates, including former Vice President Joe Biden, skipped the event altogether. Rep. Tulsi Gabbard, D-Hawaii, who is on the verge of missing the cut for the next debate, did not attend the summer meeting either, but called on the DNC to change its qualifying rules.