The strange case of a 22-year-old Soviet sailor who plunged into a Louisiana harbor twice last week became a rallying point yesterday for conservative and anti-Soviet spokesmen and a potential trouble spot for the administration before this month's superpower summit.

By the end of the day, more than a dozen members of Congress, including conservative Reps. Don Ritter (R-Pa.) and Newt Gingrich (R-Ga.), signed a letter asking that seaman Miroslav Medvid's ship, the Soviet grain freighter Marshal Konev, be detained until questions about his intentions are resolved.

Late last night, U.S. District Court Judge Louis F. Oberdorfer refused to issue a court order sought by three Ukrainian-American groups barring departure of the ship from U.S. waters.

An aide to Ritter said the signatures for the letter "came pouring in" after charges by a Ukrainian translator that Border Patrol officials returned Medvid even though he told her that he wanted to defect. Medvid was taken back to the freighter Oct. 25 after an interview with the translator, Irene Padoch, and again this week after interviews with U.S. officials.

Apparently responding to the conservative outcry, a White House official said President Reagan last week had warned the Soviets that he had "authorized on a contingency basis the use of force" unless U.S. officials were allowed to interview Medvid "in a proper environment."

The White House reiterated that it considers the case closed and that the administration feels certain that the sailor is returning home of his own will.

The Marshal Konev, meanwhile, has resumed its course up the Mississippi River and is not expected to leave U.S. waters until next week, according to its U.S. agent in Louisiana.

But as the ship steamed toward a scheduled stop at the grain elevators of Reserve, La., the clamor over the Medvid affair grew louder.

"It's either a royal foul-up at the lower levels, or it's safety engineering for the summit at the highest level," said Ritter, who is active in Ukrainian-rights matters.

"The real tragedy is that it makes us look like idiots. We're going to a summit seeking to protect freedom in the world, and we can't even provide freedom for this one desperate human being," he said.

State Department and White House officials spent much of the day denying that summit considerations affected their handling of Medvid. Some discounted the notion of Reagan as "going soft" on the Soviets.

"We don't turn a blind eye to the problems we have with the Soviets, and we certainly don't make these kinds of concessions in an area that's as important to us as human rights," State Department spokesman Charles Redman said.

Few officials deny that foul-ups occurred in the hours after Medvid's first plunge Oct. 24.

According to several accounts, the young sailor, who speaks no English, swam ashore in Belle Chasse and was greeted by a local jeweler. After several hours, he was taken to the Border Patrol, a division of the Immigration and Naturalization Service, in nearby New Orleans.

Border Patrol officers telephoned Padoch, certified by the INS as a Ukrainian interpreter, in New York late that night and interviewed Medvid with her help by long-distance, according to several officials.

"The Border Patrol asked her specifically, 'Is he asking for asylum?' and their interpretation was that she said he was not," said an INS official who asked to remain anonymous. "She has said that she said yes."

Ritter said the misunderstanding may have resulted from the poor telephone connection or because Padoch speaks with a heavy accent.

Patrol officials turned Medvid over to his ship's local agent, who chartered a skiff to return him. En route, Medvid jumped into the water again. Employes of the agent pulled him back aboard.

The State Department was not notified of the incident until late the next day, and U.S. officials did not board the ship until almost 24 hours after Medvid was returned. At the time, they said he seemed sedated.

Several U.S. officials expressed concern that Soviet authorities on the ship may have scared Medvid into changing his mind.

Three days later, when the State Department secured Medvid's release for an interview, he became nauseous, prompting suspicion that he had been drugged.

After a night's sleep, medical and psychological exams and an extensive interview by a State Department translator, Medvid persuaded U.S. officials that he wanted to return home. "We operated on the basis that there was strong evidence he wanted to stay here," Redman said. "Nonetheless, we could not detain the man against his will."