In a lengthy phone interview with Fortune late Friday, the Tesla CEO and world’s richest man said he was delighted about the change in the White House, given that Joe Biden seems determined to rein in climate change—a jolting break from Donald Trump.

“I’m super fired up that the new administration is focused on climate,” Musk says, adding that he follows Biden on Twitter and is watching his moves carefully.

Musk’s gushing enthusiasm for a Democrat is a shift.

The chief executive’s political views, which have been the subject of media speculation for years, have long leaned right on several issues. He has blocked Tesla workers’ attempts to unionize at his U.S. factories, and in the 2018 Congressional elections, he donated far more generously to Republican candidates than Democrats, including tens of thousands of dollars in a failed attempt to keep Republican control of the House of Representatives.

He clashed with California’s Democratic leaders last May, by keeping Tesla’s Fremont factory running, in violation of the state’s lockdown orders. Asked last September whether he intended voting for Trump or Biden, he again ummed and ahed.

“I’m, to be totally frank, I’m not, I mean, I think… let’s just see how the debates go…” he told the New York Times’ Kara Swisher. He said he was “socially very liberal, and then economically right of center, maybe, or center.”

But with Biden now in the White House, Musk says he is all in, and over the moon about the sea change in Washington.

“I think this is great. I feel very optimistic about the future of sustainable energy with the new administration,” he told Fortune. “Not that we should get complacent or anything, but the wind is at our back for solving the climate crisis with the new administration.”

That finally signals an irreconcilable split from Trump, after years of the two men circling each other’s orbit.

Right after Trump was elected in 2016, Musk flew to meet the President-elect at Trump Tower in New York. He was hardly the only tech titan seeking Trump’s attention. The Trump Tower gathering was a who’s who of major tech figures, including Amazon CEO Jeff Bezos, Alphabet then-CEO Larry Page, and Facebook COO Sheryl Sandberg.

But Musk appeared to have closer support from Trump than others. He pitched Trump on at least one key initiative—the SpaceX mission to Mars, which “Trump jumped at,” according to “Fire and Fury,” Michael Wolff’s book on Trump’s early White House days. And he agreed to sit on Trump’s economic advisory board and on his manufacturing jobs initiative council, remaining in place even after then-Uber CEO Travis Kalanick quit when Trump signed an executive order blocking visas from Muslim-majority countries.

Musk quit those boards only months later, when Trump announced he was pulling the U.S. out of the Paris Agreement on climate change, which 180 countries signed in 2015.

Musk says now that that move was for him the final straw. As the head of the world’s leading electric-car company, he could no longer align himself so openly with a President who eschewed global climate policy. Within hours of being sworn in as President, Biden signed an executive order last Wednesday rejoining the Paris Agreement.

But why did Musk align himself with Trump in the first place?

“I said… it is very important that we address the climate crisis and everything,” Musk tells Fortune. “I took him pretty much at his word, that he would really mean it.”

Now, he says he is waiting to get involved again as Biden begins to put in place detailed environmental policies, which are sure to be starkly different from Trump’s, including an accelerated shift to electric vehicles and renewables.

Musk says he has “not wanted to” reach out yet to Biden. “They have had a lot to do,” he laughs. But he says aides are already in informal contact with the White House. “My team, being very pro-environment, they know a lot of people that Biden is bringing in.”

Once those White House officials are ready, Musk says, “we will do our best to be helpful to the new administration with respect to climate.”