As John T. Downey tells it, "I always assumed I'd get into politics sooner or later."

His father, after all was a probate judge. And his grandfather was a state legislator and town Democratic Party chairman.

Downey collected the proper New England political credentials early on. He went to the right schools: Choate, where John F. Kennedy had gone; and Yale, where he was on the football and wrestling teams.

But at the time someone with his preparation might normally have moved on to a law school or some brokerage firm, the nation was swept up in the Korean War. Downey, with about a dozen classmates, joined the Central Intelligence Agency.

Less than 18 months later, the C47 aircraft Downey was riding in was shot down over mainland China in the Manchurian mountains.

He spent the next 20 1/2 years in a Chinese prison, a victim of the Cold War. His political career, he says, "was deferred."

Now, five years after his release, the 48-year-old Downey wants to make up for lost time. He is trying to become lieutenant governor of Connecticut.

His is a long-shot candidacy, made possible by one of the most confusing political years in recent history. More than a dozen candidates, most with wide experience in politics and state government, are seeking the Democratic nominationat the state party convention that begins Friday.

Many of them resent Downery, the political novice. "As the old story goes, he's trying to start at the top," says ine influetial Democrat. "The other guys have paid their political dues. He hasn't. All he's done is spend 21 years in a Chinese prison."

But the former CIA agent thinks the recognition he gained from international drama that once swirled around him may bring him the nomination if the party can't agree on another candidate.

"Some (people) are of my age group and seem to think, 'It could have happened to me.' A lot seem to have sympathy for what I've been through," he says.

"I know a lot of people are saying, 'This guy didn't come up through the system. He didn't serve on the city council or the state legislature," he says in his comfortanle living room, surrounded by Chinese art. "But I'm 48 and if I went that route they'd be saying 'he's too old' before long. It hardly made sense to me."

Downey is a newcomer to New Haven, and has little time to put down roots. He was released from prison in 1973 after President Nixon, who had been told Downey's mother was in critical condition, intervened. Downey entered Harvard Law School that fall.

He lives in a big house on St. Ronan Street with his wife, Audrey Lee, a biochemist from China who, coincidentally, lived about 10 miles from where Downey's plane was shot down. They met in New Haven, where she worked at the Yale Medical Center.

He practises law in suburban Wallingford with Jack Karozzella, a buddy from the Yale wrestling team, and jogs daily.

He considers the years he spent in prison "a complete waste," yet says he holds no great bitterness about them.

"I knew what I was getting into. They told me what would happen if I ever got caught," he says with little emotion. "If I blamed anyone it was myself alone for being so dumb as to volunteer."

Thhe world looked different in 1951, he recalls. "There was a great deal of romanticism about what we were going to do," he says. "Basically, we all were highly motivated. We thought the United States had its back against the wall, and we were going to save the free world.

"We saw World War III right around the cornor."

Those days seem long past to Downey. He now believes in detente with the Soviet Union, calls the U.S. policy toward mainland China "a hopeless miscalculation" for most of the last 25 years, and thinks "the CIA needs some shaping up," although he remains firm in his belief that there is need for a secret intelligence-gathering agency.

Although scores of Vietnam prisoners of war who were released about the same time as Downey developed severe emotional problems in readjusting, Downey says he avoided them.

"I was lucky," he says. "I came out relatively young, healthy and with all my marbles."

His entry into Connecticut politics was made possible when Lt. Gov. Robert K. Killian decided to challenge Gov. Ella T. Grasso in her reelection bid.

His hopes were kept alive last week when Grasso announced she would throw open to the Democratic convention the second spot on her ticket, a move unprecedented in Conneticut politics.

Downey's movtivation for running is based more on personal ambition and idealism than on any set program or series of issues.

"I've alwasys believed in public service," he says. "For me, if I'm going to do anything to make a difference in the world, it's got the be through the political route.

"I don't want to be one of those guys who goes through life making a career out of being a CIA agent who was imprisoned in China."