Ronald Reagan, stage center and playing only to cameras, is going for the old equalizer in South Carolina and is giving it his professional all.

In a small hotel meeting room, Reagan has carefully surrounded himself with retired Gen. Mark Clark, the well-known hero of World War II; Charleston ex-mayor J. Palmer Gilliard, the well-known head of Democrats for Thurmond in 1978; and Bobby Richardson, the well-known New York Yankee all-star who now is active in Christian fellowship and Carolina politics.

Reagan has done all this to counter the recent news that his GOP presidenial rival John Connally has just won the endorsement of Sen. Strom Thurmond, patron saint of South Carolina politics, and of popular former governor James Edwards. Now, bathed in endorsements of his own, Reagan drops his voice to an emotional whisper.

"I have never known such pride as I know at this moment," Reagan says, seemingly struggling to control the deep feelings welling within and blinking at tears of apparent pride welling in his eyes, "Now, while I don't feel much like it -- I'm a little bit overwhelmed -- I'll have to take some questions."

With this impassioned opening to a routine news conference, Reagan last week began his campaigning in South Carolina -- the key state in the South in this year's presidential politics, and the key to what looms as a bitter and well-financed head-to-head battle between Reagan and Connally.

Connally campaign staffers have been pointing to the south as their first real all-out effort to defeat Reagan. The Reagan campaign team has been looking to a sweep in the South as its opportunity to knock Connally out of the 1980 campaign altogether.

And both Reagan and Connally are looking to South Carolina as the crucial contest that may well decide which man captures the South -- and perhaps the GOP presidential nomination -- and which man gets to watch the 1980 Republican convention on television from the comfort of his living room, as a presidential dropout.

With President Carter mobilizing his campaign around a strong southern base, and with Sen. Edward M. Kennedy, (D-Mass.) planning to spend his resources mostly elsewhere, the South has become mainly a focus for the Republican presidential battle -- and at that, mainly the battle between Reagan and Connally.

There are other Republican candidates who will be on the ballot in South Carolina, but they are there largely as a matter of political convenience. Sen. Howard H. Baker is on the ballot, not because he thinks he will do well there or even make a major effort -- but, in the words of his campaign manager Wyatt Stewart, "because something unusual just might happen between now and then."

George Bush, meanwhile, is on the ballot in South Carolina just because Howard Baker is.

On the night of the Republican presidential debates in Iowa -- just a few days before the filing deadline in South Carolina -- Bush's advisers had been planning to bypass South Carolina, rather than pay the $1,500 filing fee. But in Iowa, Bush's campaign manager, James Baker, found himself chatting with Howard Baker's adviser, Douglas Bailey, and told Bailey he was curious about whether the senator was going to enter South Carolina, "because it is important to us to know what you're going to do there."

Apparently the Baker man took that to mean that if Baker centered, Bush wouldn't -- because the next day, on the plane back to Washington, Bailey told Bush's campaign manager that indeed, Baker would be filing just on the morning of the deadline, only to have the Bush adviser say that in that case, Bush would, too.

"It's just an insurance policy for us," said Bush's advisor, James Baker, in confirming the account. "We're in South Carolina because Howard Baker is."

With South Carolina's primary coming on Saturday, March 8, just three days before the primaries in Florida, Georgia, and Alabama, the advisers to both Reagan and Connally see it setting the tone -- and establishing the momentum -- for the entire South.

Reagan's advisers harken back to what they say was the calculable damage that Gerald Ford's razor-thin win in New Hampshire in 1976 did to their prospects in Florida just weeks later. They say a survey by their pollster, Richard Wirthlin, showed that Reagan had been trailing Ford in Florida by just two points a couple of days before the New Hamsphire election, and that he had plummeted to a deficit of 20 points just three days after New Hampshire.

Connally and his advisers have long been talking publicly about how they are pointing to the South -- and especially South Carolina -- as the key to their hopes for 1980. This recently has caused them some apparent concern.

"The emphasis on the southern states is not so much that they're early, as that they're one of Reagan's strong points," connally was quoted as saying in December in The Washington Star. "I can't realistically expect to win Iowa and New Hampshire, but if you can penetrate his strength in the South, you can destroy that aura of invincibility. By March 18 in Illnois we should have an idea of who the Republican nominee will be."

And Connally's campaign manager, Eddie Mahe, was quoted two weeks ago in the Boston Globe as saying: "We have to break through in the South in order to carry Illinois."

More recently, Connally has been trying to whittle down these high expectations, and so he declared in an interview during a swing through the South the other day that he believed he could take the nomination from Reagan even if he did not finish first anywhere. "I don't have to finish first anywere in order to win it [at the GOP presidential convention] in Detroit," he said.

At this point, Reagan generally is conceded by both those who practice politics and those who observe politics to have the best organization in the southern states of any Republican candidate. But in South Carolina, Thurmond's endorsement of Connally is viewed even by the Reagan strategists as the sort of political coup by the Texan's camp that in a single stroke dramatically narrowed Reagan's lead.

There has been no recent polling in the state.But a poll taken by one of Connally's GOP challengers showed how much the Thurmond endorsement means. Usually, when people are asked , they will say that a politician's endorsement will make "no difference" in determining how they will vote.

But this poll showed that well over 50 percent of South Carolinians responding replied that they would be more likely to vote for someone Thurmond endorsed, while just over a quarter of those responding said it would make "no difference."

There is no Democratic primary in South Carolina to compete with the Republican battle, and since the state primary rules permit crossover voting, both Reagan and Connally are hoping to attract large numbers of Democrats who will vote their way on March 8.

"We'll do well with the Democrats who traditionally vote Republican in the presidential and U.S. senate elections," said Haley Barbour, Connally's southern states coordinator. "This [South Carolina] is our best chance in many respects. It's the only state where we've got the big guns -- and in fact, we've got the only cannons around. w

"A Reagon win in South Carolina would hurt us singificantly in Florida and Georgia and Alabama."

Florida is the southernmost of the midwestern states, when latitude is measured in terms of Republican voters. Florida's Gulf Coast and much of its Orlando-Disney World citrus center is populated by large numbers of people who worked for years and raised their families in Ohio, Michigan, Missouri or Illinois and then retired to Florida, bringing their Republican ways with them.

Ronald Reagan carries an appeal through much of the Midwest that his advisers hope will serve him well in Florida. He won the politically meaningless (but media attention-getting) state convention held last fall, while Connally went all out for a win and wound up barely beating out Bush for second.

George Bush finished a strong third there, largely because of a last-minute personal effort in which he tried to chat with every single delegate to the convention and converted a number of the waverers. ("How Bush won them over is still a mystery to us," a key Connally adviser said recently.)

Connally's effort to win that convention was based not on detailed organization work, but on a costly media advertising campaign. Now the Connally people say they have learned from their bitter experience.

"We spent it all and we came away with nothing," said one of Connally's chief advisers. They are working at developing an organization.

During a recent swing through the South, Connally held no mass rallies, resorting instead to a series of smaller gatherings of several hundred people who had been invited by Connally campaign phone calls, and who were pro-Connally people. The object was to build enthusiasm within a Connally organization, not win converts from outside, aides explained.

Bush's compaign advisers, meanwhile have been putting an organization into place in Florida, hoping that strong showings in Iowa and New Hampshire will propel their man into a stronger position for Florida. Bush has some strength in the Palm Beach and lower Gulf coast areas.

"We've got two battle plans," said David Keene, Bush's political director. "We may go all-out across the state on search and destroy missions, or we may withdraw into strategic enclaves on the coast. What we do depends in part on how we've done in the earlier races."