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.”