Gov. Jay Inslee of Washington State, one of the Democratic Party’s most prominent environmentalists, endorsed Joseph R. Biden Jr. for president on Wednesday after extensive private conversations in which Mr. Biden signaled he would make fighting climate change a central cause of his administration.

Mr. Inslee, who mounted a long-shot presidential campaign of his own last year, said in an interview that he had spoken repeatedly to Mr. Biden in recent weeks and came away convinced that the former vice president was preparing to greatly expand his policy proposals for reducing carbon emissions and promoting renewable energy.

Though he and Mr. Biden clashed early in the primary season, Mr. Inslee said he was confident that Mr. Biden was “willing to aim faster and higher” on climate policy than he had indicated at that stage. Mr. Inslee said his aides were consulting with Mr. Biden’s campaign about new components to his environmental agenda.

“I am convinced, beyond the shadow of a doubt, that this will be a major driving force of his administration,” Mr. Inslee said. “I think what you’re going to see is an increased commitment to some shorter-term actions and he’s been very open to that.”

Mr. Inslee’s endorsement comes as little surprise, given Mr. Biden’s status as the presumptive Democratic nominee. The Biden campaign has rolled out a series of valuable endorsements over the past few weeks, a phase of the campaign in which public activity has been strictly limited by the coronavirus outbreak. Last week, endorsement videos from former President Barack Obama and Senators Bernie Sanders and Elizabeth Warren helped Mr. Biden stir Democratic enthusiasm and raise money during a news cycle overwhelmed by the pandemic.

Mr. Inslee has been among the most prominent governors confronting the crisis, ordering early lockdowns and other restrictions that have helped limit the damage in Washington State to a fraction of the ruin inflicted on New York, where public officials acted somewhat more slowly. The emergency has also brought Mr. Inslee into conflict with President Trump, whom Mr. Inslee has criticized for his handling of the outbreak. Mr. Trump has answered Mr. Inslee’s criticism with heated personal and partisan attacks, calling him a “snake.”

But unlike some governors who have tried to mute their differences with Mr. Trump while they are lobbying his administration for help in the fight against coronavirus, Mr. Inslee made plain in the interview that he intended to speak bluntly about what he saw as Mr. Trump’s deficiencies. He said the president had shown “no empathy for anyone but himself” and had “abused even Republican governors” who expressed dissatisfaction with the federal government’s performance.

“I’ve had to deal with a totally dishonest person in the White House,” Mr. Inslee said, predicting that Mr. Biden would be a better partner for governors in both parties.

In addition to the endorsement, Mr. Inslee participated in another ritual of Mr. Biden’s lockdown-era campaign: On Tuesday, he recorded an episode for Mr. Biden’s podcast, “Here’s the Deal,” which has previously featured conversations with Senator Amy Klobuchar of Minnesota and Gov. Gretchen Whitmer of Michigan. On the podcast, Mr. Biden alluded to numerous private conversations about climate with Mr. Inslee, as well as with former Vice President Al Gore; in his view, Mr. Biden said, confronting the climate crisis represented an enormous opportunity for the United States to create jobs “generating a fundamentally green infrastructure.”

Mr. Inslee’s vote of confidence in Mr. Biden comes at a timely moment, on the 50th occasion of Earth Day and as the former vice president continues to face grumbling on the left about the scope of his announced climate policies. While Mr. Biden has embraced a broad agenda to limit emissions, progressives have long viewed his stated timeline — reducing emissions to net zero by 2050 — as dangerously slow.

During one of the debates last year, Mr. Inslee was among those who challenged Mr. Biden’s plans as “too little, too late.”

But Mr. Biden has shown signs recently of raising his governing ambitions across the board, not only on climate, even adopting some of the messages of former rivals like Mr. Sanders and Ms. Warren in arguing that the next president will have to seek much larger social and economic change to help the country recover from the current crisis.

Mr. Inslee, who is seeking a third term as governor this November, expressed little sympathy for reluctant liberals, urging them to get on board with Mr. Biden as quickly as possible. Whatever their preferences during primary season, Mr. Inslee said, Democrats should recognize that Mr. Biden’s candidacy represents “a chance for the survival of life as we know it.”

“The faster we get in harness together, the likelier it is we will remove a man who has dedicated himself to deceit and rejection of science,” he said.

Mr. Inslee allowed that some of his enthusiasm for Mr. Biden stemmed from factors besides policy, describing the former vice president as open-minded and easy to connect with — common sentiments among Mr. Biden’s former rivals who have aligned themselves with unusual speed behind a candidate who not long ago seemed to be on the brink of defeat.

“I just enjoy talking to Joe Biden,” Mr. Inslee said. “Maybe that’s one of the reasons he’s been successful as a leader.”