Four times in the last month, the presidential candidates or their running mates have come together for nationally televised debates. Not once have they been asked about climate change. Billionaire environmentalist Tom Steyer, one of the most prominent donors to climate causes in the country, says that lack of attention to such a critical issue represents a “complete failure.”

“I think it’s a gigantic missed opportunity,” Steyer, a Democrat who has endorsed Hillary Clinton, told Fortune on Thursday. “This is, I think, the generation-defining challenge for the United States. So the idea that it wouldn’t be brought up shows a complete failure by the moderators.”

In fact, the only question about energy came during the second presidential debate, thanks to now-famous undecided voter Ken Bone, who asked how the candidates’ energy policies would balance environmentally friendly strategies with minimal job loss in the fossil fuel industry. Steyer, who said debate moderators should have asked Clinton and Republican Donald Trump about their climate solutions, thought the Democratic primary yielded fruitful debates about climate change that have since “absolutely disappeared.”

Steyer is president of the environmental advocacy organization NextGen Climate, which has spent $13 million in California this year on a voter registration drive targeting millennials—who now make up the largest share of the electorate—and $25 million on millennial engagement at hundreds of college campuses in 13 swing states.

A recent survey of millennials conducted by the Democratic polling firm Global Strategy Group for NextGen Climate showed that millennials prioritize climate change initiatives when voting.

“If it’s the No. 1 issue for young people, and there are no questions about climate and then people in D.C. wring their hands about, ‘why don’t millennials participate,’ it’s sort of like, ‘excuse me?’ It seems to me to be very clear,” Steyer said.

Trump has repeatedly said climate change is a “hoax” despite the overwhelming scientific consensus that it is real and caused by human activity. The Republican nominee has promised to save the U.S. coal industry and argued that the country should withdraw from the landmark Paris Agreement on climate change. Clinton has said she is committed to fighting climate change, delivering on the country’s Paris Agreement obligations and turning the U.S. into a “clean energy superpower.”

Steyer, who Trump recently attacked as a “radical activist,” is seen as a possible gubernatorial candidate in California in 2018. But he said Thursday that his focus at the moment is on Election Day. “I will have to make a decision, but the world will change on Nov. 8 and we’re really not sure how,” he said.

Steyer said he worries that the final presidential debate didn’t do much to give millennials a forward-looking message.

“You watched that debate last night—it was so grim. It was so grim. There was not one joke, there was not one light moment. There was a ton of anger and angst and hostility, but there was no positive moment,” he said. “If you listen to it, that’s the message: ‘You owe it to us to participate in our grim, dystopian game.’ Really? And now you wonder why they don’t vote?”

He said his priority is making sure that those millennials who have registered actually make it to the polls, which means convincing them to believe in the democratic process in a year when many have become disillusioned with the political system.

“We don’t have to convince young people that this is an important issue,” he said about climate change. “We have to convince them that their vote matters.”