George L. Hart Jr., a crusty and controversial federal judge here for almost 21 years, announced yesterday that he will assume the semiretired status of a senior U.S. District Court judge on May 16.

During an interview in his chambers yesterday, Hart, 73, said that his chronic problems with arthritis, which have left his hands gnarled and made it necessary for him to walk with a cane, "was a factor" in decision.

Hart, who comes across on and off the bench as a cross between a curmudgeon and a country gentleman, earned a reputation for hard work, conservative views and forthright rulings.He rarely issues written opinions in his cases and prefers to announce his decision immediately in the courtroom.

"I just never felt it was necessary to get into things at great length," the judge said yesterday. "I liked to hit the crux of the situation, make it as simple as possible and leave it at that. I don't think anybody ever had any trouble understanding what I'd decided."

Hart conducts himself in the courtroom with absolute authority and the air of a man who always has a clear idea of what he was about. If he ever had a moment of self-doubt, he kept it well hidden, even in what may have been the most controversial decision of his judicial career.

In 1974, after former Attorney General Richard G. Kleindienst pleaded guilty to a charge of refusing to testify accurately during his 1972 Senate confirmation hearings, Hart was criticized for giving Kleindienst a sentence of one month's unsupervised probation.

Hart said of Kleindienst's offense, committed to shield President Nixon from charges of improper intrusion in an antitrust matter, that it reflected "a heart that is too loyal considerate of the feelings of others." Asked if he had had any second thoughts about that sentence or comment, Hart said yesterday, I don't regret that in the slightest."

In the late 1960s and early 1970s, the U.S. Court of Appeals sent one case back to Hart three times before he issued a ruling that the appellate court found acceptable. That case involved attempts by antiwar protesters to ease restrictions imposed by U.S. Park Police on demonstrations in front of the White House and in Lafayette Park, across the street.

Asked yesterday if he had any regrets about his decisions as a judge, Hart said, "I can't think of anything. If I was going to regret it, I wouldn't have done it."

Joseph L. Rauh, the liberal attorney who persisted for four years to get Hart to relax the White House demonstration restrictions, said yesterday that he and Hart followed "opposing ideologies, but I think he was always fair."

Hart made his intention to assume senior status public in a memo to his fellow judges, given to them after a similar letter was delivered to President Carter yesterday. Hart's memo said he was taking the action "with more than a little regret - and sadness."

Hart said his timing was also affected by the expected arrival soon of D.C. Superior Court Judge John Garrett Penn, appointed to the District Court by President Carter. "I will transfer some of my cases to him and reduce my case load," Hart said.

At the same time, however, Hart made it clear he intends to stay active.

He will continue serving as chairman of the intercircuit committee, assigning federal judges from one district or appellate circuit to another on a temporary basis as their services are needed.

Hart said he will also continue to try some cases, although not discrimination or Freedom of Information Act litigation. "I find they're very unpleasant to try," he said.

Appoint in 1958 by President Eisenhower, Hart had been chairman of the D.C. Republican Committee before his nomination. Hart served as chief judge for the U.S. District Court here from March 1974 until his 70th birthday in July 1975, when he was require by law to relinquish the position. CAPTION: Picture, JUDGE GEORGE L. HART JR. . . . "I liked to hit the crux . . ."