Democrats won races for governor in Kentucky and Mississippi last night, smashing Republican hopes of embarassing President Carter with a series of victories in his native South.

John Y. Brown Jr., the former fried chicken king, swept to an easy victory over former GOP governor Louie B. Nunn in a bitterly fought Kentucky race. With all but 5 percent of the votes counted, Brown led by 59 to 41 percent.

With half the votes counted in Mississippi, Democrat William Winter held a similar 59-to-41 percent lead over businessman Gil Carmichael, who was trying to become the state's first Republican governor since Reconstruction.

"We have taken our state and people out of political bondage," Brown said in a victory statement. "I promise you, John Y. Brown is not going to let you down the next four years."

Nunn, the only Republican elected Kentucky governor during the last three decades, said in a concession speech that he would never run for public office again.

"The voters have done my family and me a great personal favor," he said. "What they have done for themselves is yet to be determined."

Brown, a 45-year-old multimillionaire making his first political race, campaigned as a businessman. He said he would bring a breath of fresh air into state government. In a state where politicans are normally judged on their ability to promise the sky and curry favor with courthouse politicans, Brown made few promises and ignored traditional power brokers.

He and his wife, Phyllis George, a former television sports commentator and former Miss America, campaigned as celebrities, using television, computerized voter lists and helicopters to their advantage.

Brown, who amassed a $30 million fortune as head of Kentucky Fried Chicken before selling the firm, brought Muhammad Ali and Sen. Edward M. Kennedy (D-Mass.) to the state to campaign for him.

Republicans, hoping to capitalize on anti-Carter, sentiment during the off-year elections, believed they could win at least one of the governorships, and the national party poured tens of thousands of dollars into each state.

The two contests held special implications for 1980. Mississippi and Kentucky are traditionally Democratic states that have shown a strong tendency to go Republican during presidential elections.

Carter, using his southern background to his advantage, carried the two states in 1976. But if Republicans had been able to gain control of the governors' offices, both states could have caused problems for any Democratic nominee in 1980.

It was Nunn, a seasoned veteran with a well-deserved reputation for political alley fighting, who brought bitterness into the campaign with his attacks on Brown's jet-set life style. He questioned Brown's gambling habits, his hobnobbing with socialites and bookmakers, even suggesting that Brown was quilty of Watergate-type crimes because he refused to release copies of his income taxes.

Brown retaliated with attacks on Nunn's close ties with the administration of Richard M. Nixon and suggestions that Nunn's tactics were the same as used by Hitler and the Ku Klux Klan.

What kind of governor Brown will make is something of a mystery. He will go into office with fewer commitments than any Kentucky governor in recent memory, and with a sparse knowledge of state government.

The only public office he has held was an appointment to the Governor's Economic Development Commission, and he attended about one third of that group's meetings.

But Brown will bring flair and reputation to the governor's mansion in Frankfort that will make him an instant celebrity among the nation's governors.

"I don't consider myself a liberal," he said in one interview. "I'm a moderate. I'll be a tightfisted governor."

Winter will bring to the Mississippi governor's mansion a reputation as a history buff-scholar and racial moderate as well as record in state government spanning three decades. He has previously served as lieutenant governor, state treasurer and as a state legislator.

A wiry Jackson bond attorney, he is considered a liberal in Mississippi politics, a label that normally carries with it political death.He alienated much of the state's Democratic hierarchy in 1960 by endorsing the presidency of John F. Kennedy and by urging people to go along with federal court orders later in the decade.

After two unsuccessful attempts at the governorship, he came back from political oblivion last summer to defeat incumbent Lt. Gov. Evelyn Gandy to win the Democratic nomination by telling voters he was the only candidate tough enough for the job of governor.

Republicans, buoyed by the election of Republican Thad Cochran to the U.S. Senate last fall, had targeted the state months before. By election day, the GOP had pumped $150,000 into the race, and presidential hopefuls Ronald Reagan and John B. Connally had campaigned in Carmichael's behalf.

Carmichael, 52, was considered a strong candidate. A former advertising salesman for The Wall Street Journal, he turned a $5,000 investment into a million-dollar auto dealership in Meridian. He made his name in southern politics in 1972 by winning a surprising 39 percent of the vote against then-senator James O. Eastland.

But like Winter, Carmichael had troubles with his own party.Over objections of conservatives, he advocated gun control during his 1975 race against Cliff Finch, and backed Gerald R. Ford over Ronald Reagan for president in 1976.