IN THEIR APOLOGIAS for Billy Carter's Libyan connections, the president and others in the administration, notably Attorney General Benjamin Civiletti, managed to convey the idea that registering as a foreign agent is really a ho-hum thing, basically a matter for civil jurisprudence to handle, and that the main job of the Justice Department is to try to persuade reluctant agents to be nice guys and register. Even more astonishing, the press, by and large, has accepted this interpretation of the law.

Despite President Carter's unchallenged assertion in his recent press conference that "since the 1960s" no criminal prosecutions had resulted under the act, the fact is that the Foreign Registration Act not only carries off criminal penalties but that in the 1960s -- 1963 to be precise -- there was a case in which another Democratic administration did not hesitate to move swiftly to enforce the act. I know, because I was there.

The case is remarkably similar to the Billy Carter affair in almost every respect -- except the way in which it was handled. It involved people close to the Kennedy White House, including President Kennedy's father. Instead of Libya, the foreign country was the Dominican Republic. Instead of Col. Mauammar el-Qadaffi, the head of state was Gen. Rafael Trujillo. Instead of airplanes, the matter to be influenced involved sugar imports. And instead of Civiletti, the attorney general was Robert Kennedy.

The central figure in the case was Igor Cassini, who under the pen name of Cholly Knickerbocker was a powerful Hearst society columnist. Cassini was extremely close to Joseph Kennedy Sr. He was married to the daughter of Charles Wrightsman, an oil tycoon and Kennedy family intimate. His brother, Oleg, designed Jacqueline Kennedy's wardrobe.

In the summer of 1962, as an investigative reporter for The Saturday Evening Post, I had heard rummors that Cassini was in efect selling editorial space in his column through a New York Public relations firm he had set up. I began working on the story, and subsequently heard that Cassini had been secretly representing the Trujillo dictatorship, with the bulk of his efforts being devoted to persuading the U.S. government to lift a sugar embargo that had been imposed on the Dominican Republic because of Trujillo's excesses.

After several weeks of tracking down various leads, I felt that I could conclusively show that Cassini had been on Trujillo's payroll, that he had set up still another public relations fim in the Bahamas to serve as a conduit for the money, and that he had made his initial approach on Trujillo's behalf to the president's father.

His pitch to the elder Kennedy was that a communist takeover was imminent in the Dominican Republic because of the severe economic distress the sugar embargo had caused. Joe Kennedy Sr. in turn went to his son, the president, and the upshot was that a special mission headed by Ambassador Robert Murphy was dispatched to the Dominican Republic to look into the matter.

Tagging along as part of Murphy's official entourage was none other than Cassini. In the midst of all this, as it happened. Trujillo was assassinated, not by communists but by his own dissident generals. Cassini, however, was still kept on the Dominican payroll by Trujillo's son "Ramfis," who has succeeded the tyrant.

When I had put all this together, I sought an interview with Attorney General Kennedy to inquire where the Justice Department had been all this time. It was an awkward moment for me. I had come to like and admire Bobby. I did not know what this relationship to Cassini was, but I knew how much he revered his father and that his father, dupe or not, was going to play a prominent role in my story.

I remember walking into his office and hearing him ask why our meeting was so urgent. I told him that it concerned Igor Cassini's relationship with the Dominican Republica. I remember how he raised a hand and said that the Federal Bureau of Investigation had looked into it all, and "there was nothing there."

"Well, the bureau didn't do a very good job," I replied, and for the next two hours I told Bobby what I had uncovered. After I had finished, he said simply, "Thank you."

The following day, in New York, I was visited by the first of a series of FBI agents, who appeared to be very highly motiviated about ferreting out anything I had on Cassini. I kept waiting for the other shoe to drop, to be offered an even more explosive story if I ignored this one -- or at least omitted Joseph Kennedy's name from my article.

Nothing remotely like this took place. Moreover, nobody went to Cassini to try to persuade him to register as a foreign agent. Instead he was hauled before a grand jury and indicted. He hired Louis Nizer, one of the highest-priced defense lawyers in the country, to represent him. Then, just before his trial was to begin, Cassini caved in and pleaded nolo contendre , which is an elegant way of copping a plea. He was fined and placed on probation. He also lost his syndicated column and thus his power base.

The first time I interviewed Cassini, I recall, his office walls was covered with photographs of the Kennedys. The last time I saw him, to allow him a final chance to explain away what I had discovered, all those photographs had disappeared. I think about this a lot now, and I wonder what Cassini himself is thinking today. Maybe he's thinking that timing is everything and how different it would have been if he had only hooked up with Billy Carter.

There is something else I remember. About three months after Cassini's plea, I had dinner with Robert Kennedy. The Cassini business came up during the conversation, and he said, "God knows, that was the last case I was looking to prosecute. But what's right is right."

It sounds almost quaint, doesn't it?