At his Senate confirmation hearing, Attorney General William Barr signaled an interest in confronting the power of big tech corporations like Facebook and Google. As Mr. Barr put it, a lot “of people wonder how such huge behemoths that now exist in Silicon Valley have taken shape under the nose of the antitrust enforcers.” And since taking office, President Trump has repeatedly attacked America’s biggest technology corporations, especially Amazon, Facebook and Google. Among other accusations, he has charged Amazon with ripping off the Post Office and Google with censoring conservative users. He recently criticized Facebook, Google and Twitter for “ridiculous” bias in favor of Democrats. By most appearances, Mr. Trump is a foe of big tech.

Mr. Barr, however, will find that those “huge behemoths” will continue to dominate, because, in part, the administration’s chief antitrust enforcer is their champion. On top of granting tech platforms huge windfalls in his tax reform law, Mr. Trump appointed a friend of big tech, Makan Delrahim, to lead the Antitrust Division of the Department of Justice. Mr. Delrahim has consistently promoted the interests of the biggest tech companies.

While we believe Mr. Delrahim, a veteran corporate defense lawyer and antitrust official in the George W. Bush administration, was correct on the merits in trying to stop the AT&T/Time Warner merger, that action was an outlier. In a Senate hearing in December, he boasted that he “strengthened” the Justice Department’s amicus brief program as a means of reshaping antitrust law. In its briefs, which represent the views of the federal government as an expert in antitrust law and are influential in court, the department has supported legal interpretations that would make Amazon, Google, Facebook and other dominant firms more powerful. On top of looking into possible procedural improprieties between the president and Mr. Delrahim, Congress should demand an explanation for — and put a stop to — Mr. Delrahim’s under-the-radar effort to revise antitrust law.

In the most prominent case, Apple v. Pepper, the Supreme Court will decide, any day now, whether ordinary Americans can hold tech platforms accountable. The Antitrust Division filed briefs siding with Apple. (Our organization, the Open Markets Institute, filed an amicus brief in support of Pepper.) In its briefs, the Justice Department argued that iPhone users should not be able to sue Apple for monopolizing the sale of iPhone apps. Although the text of federal law grants everyone (consumers, workers and businesses) the right to sue antitrust violators for damages, the Justice Department is seeking to restrict private citizens’ ability to hold corporate power to account. In contrast, a bipartisan coalition of 31 states filed a brief endorsing the right of all consumers to obtain compensation and be made whole.