President Trump’s Justice Department — for it is increasingly clear that the department has been reduced to an arm of the White House — has opened an antitrust investigation of four auto companies that had the temerity to defy the president by voluntarily agreeing to reduce auto emissions below the level required by current federal law.

The investigation is an act of bullying, plain and simple: a nakedly political abuse of authority.

The department is supposed to prevent companies from acting in their own interest at the expense of the public. The four automakers, by contrast, are acting in the public interest.

That the government of the United States would fight to loosen emissions standards in the face of the growing threat posed by climate change also boggles the mind. Not content to fiddle while the planet burns, Mr. Trump is fanning the flames.

Ford, BMW North America, Volkswagen Group of America and Honda struck a deal with the State of California in July. They agreed to reach an average fuel efficiency standard of at least 51 miles per gallon by 2026 . That falls short of an Obama administration rule that would have required average fuel efficiency of 54.5 miles per gallon by 2025. But it is certainly better than the goal of 37 miles per gallon favored by the Trump administration.