Billionaire environmental activist and the top individual donor of 2014, Tom Steyer, isn’t ready to get behind former Secretary of State Hillary Clinton just yet. In fact, he won’t even say that he’s ruled out supporting Vermont Sen. Bernie Sanders, Clinton’s insurgent chief rival, according to Reuters.

Steyer told the wire service that he’s not going to endorse a candidate because his goal is to raise awareness around climate change, and he still wants to hear more solutions from the presidential hopefuls before he makes a decision. He indicated that he likely won’t throw his weight behind anyone until a Democratic nominee is picked.

"We have always come out and supported the climate champion," Steyer told Reuters. "The idea that for some reason we wouldn’t do that, I’d have to understand why in hell we didn’t. Because that has been our practice always."

Steyer said Clinton needs to work on her climate plan a bit more before he could consider supporting her fully.

"I don’t think she’s fully fleshed out everything she has to say about energy and climate," Steyer said. "I think that as the campaign goes on, I would imagine she will put out more detailed plans of exactly what she thinks. I don't find what she's said inadequate, but I don't think it's complete yet."

That said, he likes Sanders's environmental plans, which include pushing away from fossil fuels and eliminating hydraulic fracking. He said that Sanders's anti-super PAC stance wouldn't disqualify the candidate in his endorsement pick.

"What Bernie Sanders is talking about, which is trying to get back to a more perfect democracy, is something that we support too. We just think that the idea of … wishing the rules were different and then pretending they were, is something which, unfortunately, probably would be disastrous from the standpoint of energy and climate," Steyer, whose money mostly goes to super PACs, said.

Landing the support of Steyer, an American hedge fund manager who has thrown the weight of his fortune behind saving the environment, would be significant. In the 2014 campaign cycle, he widely outspent other donors, giving $73.3 million to liberal causes, namely his super PAC, NextGen Climate Action. That super PAC spent $19.5 million supporting liberal candidates, according to the Center for Responsive Politics. Though it is early in the 2016 election cycle, Steyer has already forked up over $5 million to environmental advocacy groups that spend money on politics. That makes him the most generous liberal spender so far in the cycle, according to CRP.

But Clinton and Sanders aren’t the only two candidates that have been on his mind. Steyer heaped praise on former Maryland Gov. Martin O’Malley in October, saying that he went the furthest in articulating his climate plan during one of the debates. He said then that he was also impressed with Clinton’s message.