Communist leader Mikhail Gorbachev is getting a fantastic pre-summit press. His interview with Time magazine, his long seance with six U.S. senators have created a picture of astuteness, flair and mastery that has the White House wailing for equal time.

His greatest opportunity for a public-relations triumph, however, is sitting in a beach chair on a Washington street corner. Alexsey Semyonov, the 29-year-old stepson of Andrei Sakharov, is in the second full week of a hunger strike undertaken to force the Soviet government into producing his parents, whom he last saw on a Soviet videotape in July. The stream of postcards from Gorki, the town to which Sakharov and his wife, Yelena Bonner, were exiled in 1980, abruptly ceased in April.

Semyonov decided on his hunger strike on Aug. 18, the birthday of his grandmother. She lives with him in Newton, Mass., and for the first time in his life, no cable came from his parents.

"It's impossible to imagine that somehow they could have forgotten. It never happened in the 29 years I have lived. My grandmother's birthday is the main event in the family."

Semyonov sent a cable to the Soviet Embassy threatening a fast if he could not see his parents. He flew here on Aug. 30 and settled in on the corner that is more than the requisite 500 yards from the embassy. He has been living on Perrier and cigarettes ever since and has lost 21 pounds.

The worst time of the day is when he crosses the street to use a hotel's facilities and has to pass a chocolate store.

He is a tall, mild, wan young mathematician. He says in his accented English, "I don't know if it will work, but I have to do something."

He has a homemade sign beside him: "I am on hunger-strike. Soviets, let me see my parents. Free Sakharovs." He is at his post every day from 8 a.m. to 6 p.m. He took time out yesterday to escort his grandmother to the White House to see Patrick J. Buchanan.

There has been no word from the Soviets, but the response from passers-by has been gratifying. "I was only there 15 minutes when a woman came with mineral water. Another secretary who works near here typed up a petition to the Soviet Embassy. We have hundreds of signatures."

Notables visit: Jason Robards, who played Sakharov in a television film; a deputation of congressmen.

"When I came to Washington before," he observes quizzically, "I used to have to make appointments with people. Now they make appointments with me."

Semyonov thinks it is a "disgrace and a scandal for all the U.S. press" that Time had a two-hour interview with Gorbachev and never raised the question of human rights.

"The media says Gorbachev is a pragmatic, energetic leader, but if he is pragmatic he should try to do something positive by changing at least something of the human rights record in the Soviet Union. It would cost the Soviet Union nothing and would show a desire to negotiate genuinely with the West.

"I agree completely with my stepfather's thinking that the system which disregards human rights and does not allow citizens to discuss problems of the country is a danger of starting a war. It is so when the question of war and peace is decided by a small group of bureaucracy."

Semyonov subscribes to the Reagan theory of the Soviet Union as the "evil empire" -- and "if it continues for another 100 years with complete totalitarianism, it will spread."

The resolution of the Sakharov problem ties the Soviets in knots. Semyonov, who emigrated after the authorities warned they would draft him, explains that to the Kremlin, his stepfather is not just a dissident but a traitor to the establishment. Three times proclaimed a hero of the Soviet state, he was a scientist who became politicized by tests of the hydrogen bomb he created.

The Soviets know what their persecution of the saintly scientist costs them in world opinion, but they growl that Sakharov gave away state secrets, that to liberate him would be to bow to President Reagan. Sakharov is to them an agitator who misrepresents the contented Soviet people as yearning for freedom.

"It is racist and fascist to say that the Russians do not want liberty," says Semyonov. "It is in the Russian soul as in every human soul."

Gorbachev, who has demonstrated shrewdness and understanding of Western politics in the interviews, could really knock over the superpower chessboard if he let the Sakharovs go -- or at least permitted them to be seen by someone other than official Soviet cameramen.