Every fall as long as anyone around here can remember, the bolls on the cotton plants open, the sweet gum trees turn yellow, and, if it happens to be a year in which Mississippi is electing a senator, voters send James O. Eastland and John C. Stennis back to Washington.

But something strange is happening in the Magnolia State this fall: A Republican and a black civil rights leader are giving the Eastland - anointed Democratic candidate for the Senate the run of his life.

And if present trends continue, Mississppi will sent its first Republican to the Senate since Reconstruction.

There are GOP stalwarts here and in Washington who would like to read great socio-political significance into this: a rebirth of the Republican Party, a vindication of Nixon's southern strategy, and all that stuff.

Rep. Thad Cochran, the Republican nominee, dismisses it a bunch of malarkey.

"It's not a resurgence of Republicanism in the South, or anything like that," he declared here. "No question: It's a fluke, a most unusual set of circumstances that happen to benefit me. If I had to write a script, I couldn't have done a better job."

The most important "special" circumstance is Charles Evers; brother of slain civil rights leader Medgar Evers, who is running for the Senate as an independent.

Evers, mayor of the hamlet of Fayette, is a folk hero among many blacks and is in many respects the most charismatic politician in the state. Mathematically, he has a chance of winning if he could attract the support of the 37 percent of Mississippi voters who are black plus about 50,000 whites.

That's if he had the money to attract voters, and the organization to get them to the polls. He has neither.

So, the conventional wisdom here is, Evers will draw enough black votes to prevent Democrat Maurice Dantin, a former district, attorney, from succeeding Eastland, who is retiring after 36 years in the Sentate. Dantin has the endorsement of Eastland.

Result of a poll released Sunday by Cochran lend credence to this. The survey, conducted by respected Republican pollster Robert Teeter, showed Cochran with 44 percent of the vote. Dantin with 29 percent and Evers with 16 percent.

If the Dantin and Evers vote is combined it totals 45 percent, enough to give Democrat Dantin a slight edge. Alone, however, he trails Cochran by 15 percent points.

Cochran, 40, has a couple of other things going for him. He is not only the youngest and most photogenic of the three candidates, he also is the only one with any legislative experience.

With six years under his belt as a House member, he's running almost as an incumbent, saying he's the only candidate who knows his way around Washington. He has positioned himself squarely in the center of the state's conservative mainstream.

Liberal Americans for Democratic Action gave him a low 5 percent rating in each of his last two terms. He is against the Panama Canal treaties, extension of the Voting Rights Act, the Equal Rights Amendment and gun control of any type. He is for the Kemp-Roth tax cut and a constitutional amendment to force the federal government to live within its budget every year.

"Mississippi is basically a very conservative state," says Cochran. "I've preempted the right, Evers has preempted the left, and we're both squeezing Dantin for the middle."

Long-ignored Mississppi Republicans could hardly be more pleased. "Here they're about to elect a Republican senator and they can hardly contain themselves," Cochran says. "They've waited a long time for this."

The last time Missippi sent a Republican to the Senate was 1875. He was a black, Blanche Kelso Bruce, selected by a state legislature packed with carpetbaggers. The last Republican to win a statwide election. Gov. Adelbert Ames, left office in 1876.

Whit Democrats broke Reconstruction, and the Mississippi Republican Party with it. The GOP, along with black voters, virtually disappered from the state for three generations. And it wasn't until 1963 that Republicans even ran a candidate for a statewide office in the 20th Century.

The state began to break with the national Democratic Party after World War II, voting for Dixiecrat Strom Thurmond over President Harry Truman in 1948. GOP presidential candidates carried the state in 1964 and 1972. Cochran and another young Republican, Trent Lott, were swept into Congress with the Nixon landslide in 1972, and have remained.

Last Spring, Cochran coasted to an easy win over state Sen. Charles Pickering in the GOP Senate primary, the first ever held in the state.

Dantin, 48, has a much more difficult time, defeating two better-known candidates, Gov. Cliff Finch, who formed a "blackneck-redneck" coalition to win the governorship in 1975, and former Gov. William L. Waller.

The victory gave Dantin a giant-killer image and a quiet confidence that he can overcome Cochran during the four weeks before election day.

"Nobody thought we had a chance in the primary or the runoff," Dantin says. "All the polls had us running third or fourth."

He disputes the accurancy of Cochran's self-serving poll, but says he has no polls of his own 'All I know is what I see, and I'm getting a tremendous response everywhere I go," Dantin said as he campaigned in Pascagoula last week.

However, he admits that he and over Cochran agree on most issues, and that the biggest thing going for him during the June runoff was voters' negative feelings about Finch.

He's running as a Democrat's Democrat, trying to capitalize on traditional party ties. His chief pitch to voters is that a Democrat can do more for Mississippi in the Senate.

He has the public support of the state AFL-CIO and received a tearful endorsement from Eastland. But he's tried to stay clear of any ties to Jimmy Carter, who narrowly carried the state in 1976, and has refused to invited any out-of-state Democrats to campaign for him.

For months, Cochran hammered mercilessly against Carter, but he has cooled his rhetoric since the Camp David summit, which he initially described as "a media event" of no real significance.

Evers, 55, is by far the most interesting candidate in the race. The forceful orator steals the show every time the three make a joint appearance. A longtime force in civil rights activities, the former state NAACP field director is making an outright appeal to conservative whites this fall.

He would crack down on fathers who desert and throw their families onto the welfare rolls. He is against busing to achieve school integration. He thinks unions are bad for small Mississppi towns. He wants constitutional amendment to put prayer back in the schools.

But the biggest Evers pitch in that he can do more for Mississippi than his white opponents. Cochran, he says, didn't "bring home the bacon" during six years in the House and won't in the Senate. Dantin, he says, is so inexperienced "it will take him six years in Washington to find the front door."

Mississippi has had " the worst reputation of any state, and we've earned it." but is changing into one of the most integrated states in the union, Ever says, and nothing would signal this chang more than the election of a black to the Senate.

"You send me up there and it will change your image overnight," Evers says. "I'm the only candidate who can say that."