Democratic National Committee chairman Tom Perez on Tuesday had a simple explanation for rejecting Washington Gov. Jay Inslee’s request for a climate-focused primary debate: If the DNC said yes to this one, they’d have to say yes to every other candidate’s issue-specific debate ideas.

“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 Medium post on Tuesday. “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 has been under fire for rejecting Inslee’s pitch for a Democratic debate on climate change. The governor had aggressively petitioned both Perez and the DNC to host such an event, a call that was also initiated by a number of progressive groups eager to have a discussion about what Perez has called an “existential threat” to the world.

The progressive argument against Perez is that a debate around climate change does not amount to a simple “pet issue,” but rather one that voters and candidates alike have prioritized as an all-encompassing threat impacting people’s everyday lives.

“We have received more than 50 requests to hold debates focused on these important issues and many others,” Perez wrote on Tuesday. “And we knew it would be unfair and unrealistic to ask the candidates to participate in so many.”

He and the DNC have said that they told media partners to make sure they ask questions on climate change during the debates—questions that were woefully absent in debates during the last presidential cycle.

“Climate change is an urgent threat to our nation and our planet,” Perez said. “It imperils our children and grandchildren’s future, and it disproportionately affects our most vulnerable communities. That’s why, 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.”

Perez has not appeased many who have sought answers on why there can’t be a special accommodation made for such an issue. According to a Tampa Bay Times report on Sunday, Perez told activists who confronted him about the lack of the debate, “It’s just not practical,” adding that “as someone who worked for Barack Obama, the most remarkable thing about him was his tenacity to multitask, and a president must be able to multitask.”

Activists plan on bringing more than 200,000 petition signatures collected by Greenpeace USA, Sunrise Movement, US Youth Climate Strike and more to DNC headquarters on Wednesday morning. In advance of that, activists with Greenpeace appear to have gotten former Vice President Joe Biden to support having a climate debate.

Inslee, for his part, was unmoved by Perez’s Tuesday explanation.

“The climate debate isn’t about one candidate, it’s about our one planet,” he said. “Hundreds of thousands of grassroots activists, many progressive partner organizations, 9 state party chairs, over 50 DNC members, and 14 Presidential candidates have called for a climate debate. The call for a climate debate started with—and has always been led by—grassroots activists.”

The governor added: “I will not back down: We need a full debate of this existential crisis.”