Mississippians have proved again that they are not ready to elect a woman governor and, in doing so, have raised Republican hopes for picking up a governorship in the Deep South.

Former lieutenant governor Evelyn Gandy lost her second consecutive bid to become the state's first woman governor when she was defeated in Tuesday's Democratic runoff election by Attorney General Bill Allain, who won 52.3 percent of the vote.

"I think sexism played a role in her defeat," Beverly Hogan, Gandy's state field coordinator, said Tuesday night in a remark echoed by many Democratic Party leaders. "We're still being held back by sexism and racism in Mississippi. If we expect to grow, we're going to have to overcome it."

Gandy's defeat sets up a gubernatorial race involving three major candidates--Allain, Republican Leon Bramlett and independent Charles Evers, a well-known civil rights leader making his third try for the office.

If no candidate receives more than 50 percent of the vote in the Nov. 8 general election, state law requires that the new state house of representatives decide the issue. Democrats have dominated that chamber for years.

Allain, 55, carried almost three-fourths of Mississippi's 82 counties with a populist campaign focusing heavily on his victories as attorney general over utility companies seeking rate increases. However, his anti-establishment rhetoric and liberal views make him suspect among many Democrats.

Bramlett, 59, a former state Democratic chairman who switched parties in 1976, tried to capitalize on this today, appearing at a news conference with Democrats who have endorsed his candidacy.

"My campaign is an open coalition of Democrats, Republicans and independents," he said. "I hope 1983 will be the year in Mississippi when everyone votes the man and not the party."

In a veiled reference to Allain's campaign style, Bramlett added: "I will present a positive program for Mississippi, not just run against scapegoats or symbolic straw men."

No Republican has won the Mississippi governorship since Reconstruction, and Bramlett's strategists concede that he faces an uphill fight. His prospects rose last week when Evers, 60, former mayor of Fayette and brother of slain civil rights leader Medgar Evers, announced that he would run as an independent.

Evers' three unsuccessful statewide campaigns have severely damaged his credibility, and he has been widely criticized by blacks for entering this race because they fear he will draw votes from Allain, who fared well among black voters on Tuesday.

State Democrats remember that Evers' bid in the 1978 U.S. Senate race resulted in election of Republican Thad Cochran. Randall Patterson, executive director of the state Democratic Party, said 1978 "attested to what a viable black candidate does to the Democratic coalition. With him Evers in there, the coalition suffers."

Some Republicans, angry at suggestions that GOP interests lured Evers into the race, argued that Evers could damage Bramlett.

"Evers is a charismatic guy. He fires up the electorate, black and white, and brings out a lot of people who wouldn't vote otherwise," said a GOP officeholder, who asked not be identified. "Bramlett needs a low-turnout, low-interest race to win."

Only three governorships are being contested nationwide this year, and Bramlett said he expects to receive a great deal of help from the national party. President Reagan has visited the state on Bramlett's behalf, and Bramlett said today, "If we decide we can't do without him, we'll ring the bell for him again."

Bramlett is an attractive candidate for the Republicans. A farmer and businessman from the Mississippi Delta town of Clarksdale, he was an All-America end at the Naval Academy in 1944 and 1945. He served as Democratic state chairman from 1968 to 1972, and made a short, unsuccessful run for the GOP gubernatorial nomination in 1979.

Bramlett's strategists are counting on a backlash against Allain. "Allain is a controversial guy," said Charles Black, a Bramlett campaign consultant. "When you only win with 53 percent of the vote, that means there are a lot of people out there not enamored of you."

Allain told reporters today that he has no intention of retreating from the themes that helped him capture the Democratic nomination. "Together we can get fair utility rates, cut down crime and keep nuclear waste out of the state," he said.

Allain is a somewhat unusual candidate for Mississippi. In this heavily Protestant, Bible-belt state, he is a divorced Roman Catholic, who attended the University of Notre Dame. His first elected public office was attorney general.

A long-time assistant attorney general, he built a political base during the last four years by frequently visiting county courthouses. He attracted widespread attention by opposing and stopping a $68 million rate increase sought by Mississippi Power & Light Co., which serves about half of the state, and customers received refund checks averaging about $100 each this summer.

Gandy, 62, who said she had dreamed of becoming governor since her high school days in Hattiesburg, is a veteran of 35 years in Mississippi politics and seven statewide campaigns.

She has been state treasurer, insurance commissioner, lieutenant governor and welfare commissioner.

Although she lost the Democratic runoff four years ago to Gov. William F. Winter, Gandy entered this year's race as acknowledged front-runner and was the top vote-getter in the Democratic primary on Aug. 2, over Allain and Mike Sturdivant, a wealthy planter and businessman.

Allain did not refer directly to Gandy's gender in his campaign and portrayed himself as an aggressive fighter for the common folk who would keep big-money interests from "messin' with Mississippi."

Gandy, who had ignored the gender issue during her loss to Winter, tackled it head-on this year, telling audiences, "I worked hard to earn my own way.

"Don't let anyone tell you Mississippi won't elect a woman governor," she said during the campaign.

Gandy today issued a brief statement congratulating Allain. She thanked her supporters and said: "Ours was a message of hope and progress for Mississippi, and we fought a good fight."