WASHINGTON — In 1981, a Justice Department prosecutor from Washington stopped by to see Jeff Sessions, the United States attorney in Mobile, Ala., at the time. The prosecutor, J. Gerald Hebert, said he had heard a shocking story: A federal judge had called a prominent white lawyer “a disgrace to his race” for representing black clients. “Well,” Mr. Sessions replied, according to Mr. Hebert, “maybe he is.” In testimony before Congress in 1986, Mr. Hebert and others painted an unflattering portrait of Mr. Sessions, who would go on to become a senator from Alabama and now, according to numerous sources...