Thousands have come out to the sheriffs' boys ranch in rural Lowndes County, Miss., for free barbeque and cokes and Ronald Reagan, but before things can get under way, the Rev. Robert Penny must give the invocation.

". . . We thank you, oh God, for one who has risen in our midst to declare traditional biblical values. We thank you for his witness and for his patriotism, for his love for country and for God. We pray your blessings upon this candidate, Gov. Ronald Reagan. We pray that he may be blessed in all of his efforts to serve our nation.

". . . We thank you, oh God, for his stand for the many wonderful values . . . which are now being stomped afoot in our land. We pray that you would bless him as he seeks to affirm the dignity also of human sexuality against those who would pervert this wonderful God-given gift. God, we thank you for one who has arisen to declare your faithfulness, and the fact that we should acknowledge you in every place in our land, even the classrooms. . . . This we pray in the name of Jesus Christ, the great king and ruler of all nations. Amen."

On stage, Ronald Reagan prayed with him, carefully humble but appreciative.

In this region where the presidency may be decided, the Rev. Pennys have come to be regarded with political reverence. "This election could be decided on what the preachers say in the pulpits on that final Sunday," one Reagan southern strategist remarked.

In 1976, Jimmy Carter won the presidency because he carried all of the southern and border states except Virginia -- and his success in the region was due in large measure to the fact that he enjoyed the support of more white southerners than any other recent Democratic nominee. He didn't carry the white southern vote, but he did better among southern whites and did Lyndon Johnson or Hubert Humphrey or George McGovern. (Southern blacks voted for Carter in overwhelming numbers -- but they had given Humphrey and McGovern about the same percentage of support; and, contrary to common perception, the turnout of black voters in the South in 1976 was about the same as the 1972 turnout and even smaller than that of 1968.)

But this year, Carter's share of that white southern vote has slipped a bit -- some of it going to Reagan and some of it still undecided -- and the shift could prove crucial. The election is quite close now in at least seven states in the region, with Reagan holding a significant lead in the two largest, Florida and Texas, and with Louisiana, Mississippi, South Carolina, Tenessee, Kentucky, and perhaps even Missouri, all within striking distance for the Republican challenger.

The Moral Majority, the right-wing Christian political crusade, has done some heavy voter registration crusading of its own, registering pro-Reagan Christians in especially large numbers in Tennessee (the Chattanooga area), Alabama, and the north Florida's panhandle. Their actions in these areas have brought new concern to the Carter camp.

Reagan, meanwhile, spent virtually all of last week trying to broaden his appeal with those white southerners who were Carter's in 1976. He took care to bestow grateful appreciation upon his allies as he rolled through Jimmy Carter's once-solid South.

He was prayerfully appreciative of the Rev. Penny's invocation. And he was chuckingly appreciative of former Mississippi governor John Beli Williams, a right-winger by every definition. In praise of Reagan, Democrat Williams declared at the rally that the 1980 choice is between "a man who advocates Jeffersonian philosophy but he's having to run on a Republican ticket, against a man who put aside Jefferson and picked up Karl Marx."

Reagan's prospects in the South may come down to events such as that one in Lowndes County. There, the Reagan rally appeared to be a huge success on that sprawling ranch for the orphaned and unwanted that is run by Mississippi sheriffs, and that so far houses only white boys.

The Republican campaign organization, Commitment '80, had agreed to put on what would be called a fund-raiser for the boys ranch in exchange for using the facilities and aegis of the Mississippi sheriffs for their rally -- the local sheriff, in fact, declared Reagan an honorary deputy. It turned out to be a rare event -- a fund-raiser without an admission charge. "But they did pass the hat, which makes it a fund-raiser," says Buddy Bynum, Reagan's spokesman in the state.

Thousands turned out for the free food and drink, among them crucial Reagan targets such as Mark Uithoven, 30, a clean-cut and well-dressed plant manager for a firm that makes hospital supplies. He is asked whom he voted for in 1976.

"Carter -- I ain't going to lie about it," he says. Why Carter? "I voted for him because he was a good ole southern boy. But he just didn't have it."

Uithoven is a convert for Reagan now. But he adds that he does know a number of people who are for Carter. "They're blacks, mostly -- a lot of them who work for me are for Carter."

Next door in Louisiana, it is Carter who is benefiting from the support of a former governor. Carter's campaign in Louisiana is headed by silver-haired Edwin Edwards, a Cajun with a high-gloss, high-fashion and an even higher popular standing in his state. Edwards has never been very high on the likes of Jimmy Carter, his former fellow-southern-governor. As he told the New Orleans Times-Picayune, one of his least favorite newspapers, just the other day: "I am enthusiastic about him [Carter] the way I feel when I pick up the Times-Picayune -- there ain't no other action in town, so I buy the Times-Picayune."

But Edwards nevertheless is taping television commercials for Carter and giving interviews extolling in his inimitable way the virtues of Carter as compared to Reagan. "Between these two characters," he said in an interview, "whether you are concerned about the economy or war and peace or race relations or energy, President Carter is head and shoulders above this man Reagan. . . .Sure Reagan has appeal down here; but cocaine has an appeal too, only it's mindboggling. Reagan is just a euphorism for George Wallace."

Carter, meanwhile, is still counting heavily on the hope that his own special appeal still carries weight down home.It is the appeal that Carter laid on liberally last week in a torchlight rally in New Orleans' Jackson Square.

"We southerners know," he said, his syntax suffering somewhat from a long day on the trail, "that no southern president has served from the Deep South since 1844 when James Polk was elected. We're going to keep the hands one the reins in Washington with southerners who brought our nation together in a unified way, to have a better future for us all."

EPILOGUE: After that rally on the sheriff's ranch, the Rev. Penny, of the Presbyterian Church of Columbus, Miss., is asked about whether he thinks it proper to invoke the Lord's name in Reagan's behalf and to direct the prayers of his audience toward a Reagan victory.

"Yes," the reverend tells a reporter. He says he is not actually endorsing Reagan, just expressing his own beliefs. "Freedom of religion . . . freedom of speech," he explains. And while he says he is not a member of the Moral Majority, "I do appreciate what they are doing."

Then the preacher interrupts to ask a question of his own. "Because we are beginning to stand up for these issues, are you saying that we should lose our tax exemption?" The reporter says he had not even mentioned taxes or exemptions.

"No," the preacher says, "but I thought that was where this thing was going."