OXFORD, MISS. -- The decision by Sen. John Stennis not to seek reelection means bad news for Jesse Jackson.

As the state with the highest percentage of black population, Mississippi is important to Jackson's southern strategy -- the key to maintaining his credibility as a candidate. Jackson ran well in the south in 1984, partly because whites stayed home in droves.

The Stennis decision will make things different next year in Mississippi. Traditionally, the Democratic primary for congressional elections is held here in August. But to save the cost of duplication, the Mississippi legislature advanced the date to coincide with next spring's Super Tuesday presidential nominating primary on March 8.

Because Mississippi elects its governor, legislature, and local officials this coming Tuesday -- and since all five incumbent congressmen were expected to seek reelection without serious opposition -- the March primary would have pulled few voters to the polls. Thus, Jackson could have anticipated strong black support and a realistic chance for a first place finish.

But no longer. A hotly contested race for the Stennis seat began developing hours after he announced his decision on Oct. 19. Before sundown, 4th District Congressman Wayne Dowdy, who in 1981 built a populist coalition around blacks and organized labor to win a special election, became a Senate candidate and opened up a congressional seat.

Secretary of State Dick Molpus shows every sign of entering the Senate race soon after he gets reelected Tuesday, as he surely will. Molpus, 33, is allied with Ray Mabus, a 39-year-old state auditor and Harvard Law graduate who is expected to be elected governor Tuesday. Both are reformers who returned to Mississippi after working in Washington to take key positions on the staff of progressive Gov. William Winter. They represent a new generation of political leadership in the state.

Several others may enter the Senate race, including Hiram Eastland, a cousin of the late segregationist senator James B. Eastland. Like Mabus and Molpus, Hiram Eastland also has spent time in Washington and he boasts that his friends there include Joseph Kennedy II, the new congressman from Massachusetts and son of the late Robert F. Kennedy.

Arthur Schlesinger Jr. has recorded the special private relationship that existed between Jim Eastland and the Kennedys. If Hiram Eastland runs, he no doubt expects his public identification with the Kennedy name to help him among black voters and his Eastland name to appeal to whites.

What all this means is that Mississippians will have much more to vote for in the March primary than the choice of a Democratic nominee for president. White voters will pour out to vote -- but not for Jesse Jackson.

The situation in Mississippi exposes Jackson's inherent weakness. In several other southern states where the primary date has been advanced for all offices, local and legislative races will boost white turnout. In addition, the Democratic presidential race next year will mean something to southern whites, many of whom stayed home in 1984 because they already had decided to vote for Ronald Reagan in November.

The most likely beneficiary of these changes is Albert Gore Jr., who is running as a southerner strong for defense. In Mississippi, whose northern border joins Tennessee, being a neighbor gives him further advantage.

In addition, Gore also may chip away some of Jackson's black support. Some of Jackson's strongest supporters among black elected officials candidly say they want him in the race because he is the only candidate who articulates issues of special importance to blacks.

But as pragmatists they also want to back a candidate who can win. And they remember their comfort with Jimmy Carter as an understanding white southerner in the White House. Gore's connections to potent black political leaders in Memphis likely will spill down into Mississippi.

But Gore first must make a sufficiently strong showing in Iowa and New Hampshire to demonstrate that he has strength outside the south. If he falters, the Democrat best positioned to gain would be Paul Simon, who is developing organizational strength in Iowa. A win there would propel him out of obscurity.

In 1984, Simon defeated incumbent Republican Charles Percy in Illinois without Jackson's endorsement, but still got more than 90 percent support from blacks. Simon's refusal to meet with Jackson to discuss an endorsement will sit well with whites in the south. And his strong support from Illinois blacks, pro-civil rights record, and his commitment to federal education programs would appeal to those blacks who are looking for a viable contender rather than a symbolic candidate.

Who gains may be in doubt, but Jesse Jackson is the clear loser in the post-Stennis Mississippi scenario.

Jack Bass, an associate professor of journalism at the University of Mississippi, is coauthor with Walter DeVries of "The Transformation of Southern Politics."