President Trump’s policies could threaten many big green energy projects in the coming years, but his election has already dealt a big blow to an ambitious California effort to replace thousands of diesel-fueled trucks with battery-powered semis.

The California plan, which has been closely watched by other states and countries, was meant to take a big leap forward last year, with a requirement that some of the more than 30,000 trucks that move cargo in and out of ports start using semis that don’t emit carbon dioxide.

But after Mr. Trump was elected, California regulators withdrew their plan, which required a federal waiver that the new administration, which is closely aligned with the oil industry, would most likely have rejected. That leaves the state unable to force trucking businesses to clean up their fleets. It was a big setback for the state, which has long been allowed to have tailpipe emission rules that are stricter than federal standards because of California’s infamous smog.

Some transportation experts said that even before Mr. Trump’s election, California’s effort had problems. The batteries that power electric trucks are too expensive. They take too long to charge. And there aren’t enough places to plug the trucks in.

“It was excessively ambitious,” said Daniel Sperling, a professor at the University of California, Davis, who specializes in sustainable transportation, referring to the program that made truckers buy green rigs.

California officials insist that their effort is not doomed and say they will keep it alive with other rules and by providing truckers incentives to go electric.