Billionaire environmentalist Tom Steyer explains what took him so long to endorse Hillary Clinton

Hillary Clinton picked up the endorsement of billionaire environmentalist Tom Steyer on Wednesday morning, hours after she was projected to win California’s primary.

“Today I am endorsing @HillaryClinton for President,” Steyer, founder of the California-based NextGen Climate advocacy group, announced on Twitter. “Now is the time for us to unite and defeat Trump. #ImWithHer”

Steyer, who was one of Clinton’s early backers during her 2008 campaign, hosted a fundraiser last summer for the former secretary of state. But he never came fully on board until this week, when Clinton locked up her party’s nomination.

Why the delay in endorsing her this time around?

“We really wanted to play a very specific role in this campaign, which was to make sure climate and energy were a central part of the conversation,” Steyer told Yahoo News on Wednesday. “So what we did was try and challenge every single candidate for both parties to come up with solutions to the climate crisis.”

Today I am endorsing @HillaryClinton for President. Now is the time for us to unite and defeat Trump. #ImWithHer

— Tom Steyer (@TomSteyer) June 8, 2016

As for the climate, Steyer said the choice between Clinton and presumptive Republican nominee Donald Trump is crystal clear.

“We couldn’t imagine a starker choice between candidates than the ones we’re getting here,” Steyer said. “[Trump] is not a policy-driven candidate. He is an attitude-driven candidate, and his attitude is pretty much belligerence.”

Last month, Trump outlined an energy policy plan that would roll back environmental protection regulations enacted by President Obama, expand fossil fuel exploration and revive the nation’s coal industry. During a recent campaign stop California, Trump, who once described global warming as a Chinese hoax, declared, “There is no drought.”

“As far as I can tell, he’s shown no understanding of the topic, no awareness of what’s going on and no ability to take information and turn it into a forward-thinking policy,” Steyer said, adding that a President Trump “would absolutely be a disaster for the world.”

Steyer wouldn’t say how much money he’s willing to spend to make sure that “disaster” doesn’t happen.

“There is just a dramatic choice between these two presumptive candidates,” he said. “And it is really important that people rally around Secretary Clinton.”

Clinton’s main primary rival, populist Vermont Sen. Bernie Sanders, continues to stay in the race despite the fact that Clinton now has enough delegates to be the nominee. Many observers have speculated that Clinton will need Sanders’ support to ultimately unify Democrats behind her candidacy.

Steyer, a former hedge fund manager, dismissed concerns that another “big money” endorsement of Clinton could hinder her efforts to attract supporters of billionaire-averse Sanders, who has railed against Citizens United — the 2010 Supreme Court ruling that opened the doors for so-called super PACs that could spend unlimited sums during elections.

“We’ve always been opponents of the Citizens United decision,” Steyer said. “Our role is to empower voter-to-voter contact to ensure there is the broadest possible democracy.”