The Senate Judiciary Committee is expected to approve today President Reagan's nomination of D.C. Appeals Court Judge Stanley S. Harris to be U.S. attorney for the District of Columbia. Staff members said that they anticipate that the full Senate will confirm the nomination by Wednesday.

Harris, who testified briefly at confirmation hearings yesterday, said that he had selected Joseph diGenova, chief counsel to the Senate Rules Committee and a former assistant prosecutor, to be his principal assistant. DiGenova, who also worked as a law clerk at the appeals court, said yesterday that he is "absolutely delighted" to be returning to the federal prosecutor's office.

If he is confirmed by the Senate, Harris is expected to take over the U.S. attorney's job by Jan. 15. Harris, a conservative Republican, succeeds Charles F.C. Ruff, who was appointed to the top prosecutor's job in 1979 by President Carter.

At his confirmation hearing yesterday, Harris was praised by committee chairman Sen. Strom Thurmond (R-S.C.) and Sen. Charles McC. Mathias Jr. (R-Md.), who were the only committee members present for the 45-minute session. The single witness was Ronald Drake, president-elect of the Logan Circle Citizens Association, who said he wanted the federal prosecutor's office to make a stronger effort to cut down on drug and prostitution traffic in his neighborhood. Drake also noted that he supports Harris. There were no opponents to the nomination at yesterday's hearing.

During brief questioning by Thurmond, Harris said he believes there is a "real perception in the community" that bail laws favor defendants; that he would like to see changes in laws about illegally seized evidence which he feels have "mired down the courts" in technicalities, and that he believes that on balance, the death penalty would be a "deterrent" to crime.

In response to questions by Mathias, Harris said that he was concerned about violent crime, that he "certainly will" make drug traffic an important issue while he is the federal prosecutor, and said he hadn't studied the question of gun control "a great deal" and declined to comment on the issue, except to note that the District already has a gun-control law.