President Carter has selected Rep. Abner J. Mikva (D-Ill.) and Assistant Attorney General Patricia M. Wald to fill two seats on the U.S. Court of Appeals here. The choices could help the influential D.C. circuit regain a place as one of the most activist judicial forums in the country, legal observers said yesterday.

The two lawyers -- one a five-term congressman who barely won reelection from his silk stocking district because of his liberal views, the other a lawyer who had been involved in a wide range of public policy issues before joining the Justice Department two years ago -- must await clearance from the FBI and screening by the organized bar before Carter formally can nominate them for the Judgeships.

The selections of Mikva, 53 and Wald, 50, to the two newly created seats on the court were made during a meeting Wednesday afternoon between Carter and Attorney General Griffin B. Bell. Mikva, informed by Bell, released the news of the choices.

If confirmed by the Senate, the two will join a court responsible not only for cases brought up from the U.S. District Court here, but also for appeals from decisions by federal regulatory agencies. In that role, the appellate court's decisions have a vast impact on the nation's economic life.

The court under former Chief Judge David L. Bazelon earned its activist reputation during the past two decades for a series of decisions that expanded the rights of the mentally ill, alcoholics and criminal defendants. It also is the place where Warren E. Burger gained his judicial experience before he was named by President Nixon as chief justice of the U.S. Supreme Court in 1969.

Burger filed strong dissents to the majority opinions of the court during more than 13 years on the appeals bench.

Lawyers here said the expected addition of Mikva and Wald to the court will renew its activist image, which was changed somewhat by the more conservative philosophies of three Nixon appointees and a shift in the kinds of cases brought to the U.S. appeals court. That resulted from an upgrading of the local court system and the transfer of most criminal and civil cases from the U.S. District Court to the D.C. Superior Court.

When Congress enlarged the federal judiciary last year, it expanded the appeals court here from nine to 11 members.

One lawyer with wide experience with regulatory agencies, commented that the appointment of Wald and Mikva "will have some great impact on the Federald Trade Commission," currently the most controversial agency in Washington. FTC decisions on the regulation of business and professional practices are sure to be appealed to the U.S. courts.

Another close observer of the U.S. Court of Appeals said Mikva's legislative experience will be "a real enrichment" to the court in helping it interpret what Congress meant by the laws it passed.

Throughout his career in Congress Mikva has been a strong supporter of Liberal social programs and tax reform legislation so much so that he was in constant political trouble in his home district, an affluent suburb of Chicago. He barely won reelection last November and in 1976 his victory margin was 201 votes out of 213,000 cast. In 1972 he lost his seat by 2,500 votes.

Wald, who would be the second woman sitting on a federal appeals court, gained the support of Attorney General Bell through her work as assistant atorney general for legislation.

She was director of the Mental Health Law Project here before joining the Justice Department. Prior to that she served on President Johnson's D.C. Crime Commission, was a staff attorney for Neighborhood Legal Services here, worked on the American Bar Association's groundbreaking new standards for juvenile justice, was on the forefront of bail reform and wrote a book on drug abuse. She also was on the board of trustees of the Ford Foundation.

Moreover, she has a reputation for being able to hammer out consensus positions -- a key factor in a court in which most decisions are handed down by three-judge panels.

She is married to Washington attorney Robert Wald and they have five children. She and her husband met at Yale Law School, where they were classmates.