WASHINGTON — President Trump on Wednesday posted on Twitter an aggressive defense of his unprecedented move to abolish California’s legal authority to set its own standards on climate-warming automobile emissions.

Trump was in Los Angeles for a fund-raiser when he boasted about the move, which California officials and environmental advocates have assailed as an illegal attack on states’ rights and on a major policy designed to fight climate change.

“The Trump administration is revoking California’s Federal Waiver on emissions in order to produce far less expensive cars for the consumer, while at the same time making the cars substantially SAFER,’’ Trump wrote. He said the change would lead to increased auto production and claimed newer cars would be “extremely environmentally friendly.’’

California officials said they would sue to block the move .

“Our message to those who claim to support states’ rights is, ‘Don’t trample on ours,’ ’’ said Xavier Becerra, attorney general of California. “We cannot afford to backslide in our battle against climate change.’’

The Trump administration is expected Thursday morning to formally revoke California’s authority to set auto emissions rules that are stricter than federal standards, taking a major step forward in the administration’s wide-ranging attack on efforts to fight climate change.

Andrew Wheeler, head of the Environmental Protection Agency, and Elaine Chao, transportation secretary, are scheduled to announce the formal abolition of the waiver.

That would have national significance. Tailpipe pollution is the United States’ largest source of planet-warming greenhouse gas pollution, and California, with 35 million vehicles, is the nation’s largest auto market. California has historically set stronger pollution standards than the federal government, and many of those standards have influenced national and international policy.

Thirteen other states follow California’s tighter tailpipe greenhouse gas standards, together representing roughly a third of the national auto market.

The president’s three Twitter posts sought to defend the revocation by citing details of the plan. As the political battle over climate emissions has intensified, Trump’s interest in the policy details has deepened, said a person familiar with the matter.

“If you look at the whole tweet thread, this is remarkably specific,’’ said Barry Rabe, a professor of public policy at the University of Michigan. “It almost moves into policy-wonk territory. If he’s doing this himself, he’s on his policy game.’’

Still, Rabe and other experts said Trump’s assertions, while using the language of emissions policy, veered from the facts on several significant points.

Several analysts, for instance, disputed Trump’s assertion that weakening emissions standards would improve highway safety.

“The president’s claim that high fuel economy negatively affects safety is baseless,’’ said Shannon Baker-Branstetter, coauthor of an August Consumer Reports analysis concluding that the Trump administration’s rollback of fuel economy standards would have no statistically significant effect on highway safety.

Legal experts said that if Trump’s move was ultimately upheld by the Supreme Court, it could permanently block states from regulating greenhouse-gas pollution from vehicles — a major setback for efforts to control climate change.

The administration’s plans have been further complicated because major automakers have told the White House that they do not want such an aggressive rollback. In July, four automakers formalized their opposition to Trump’s plans by signing a deal with California to comply with tighter emissions standards if the broader rollback goes through.