WASHINGTON — Two Democratic senators asked the Justice Department’s internal ethics watchdog on Thursday to investigate Attorney General William P. Barr for blessing a secret program at the Drug Enforcement Administration more than a quarter-century ago that went on to collect billions of records of Americans’ phone calls.

The senators — Ron Wyden of Oregon and Patrick J. Leahy of Vermont — accused Mr. Barr, during his first stint as attorney general, of approving “an illegal, bulk surveillance program” in 1992 “without conducting any legal analysis” of it.

“Mr. Barr’s authorization of this sweeping surveillance program without requiring, at minimum, an appropriate legal analysis, was not consistent with his oath to support and defend the Constitution and it likely amounted to professional misconduct,” the senators wrote to the Justice Department’s Office of Professional Responsibility.

A spokeswoman for Mr. Barr did not respond to a request for comment.

The letter comes at a time when Democrats have frequently criticized Mr. Barr for aligning himself with President Trump’s attacks on the F.B.I.’s Russia investigation and for supplying legal defenses for aggressive White House moves, like invoking emergency powers to spend more on a border wall than Congress appropriated and defying subpoenas.