It was a scene from campaigns past: bands playing "The Battle Hymn of the Republic," crowds cheering madly and Ronald Reagan back in form, railing against the evils of big government.

He was relaxed, confident, almost young again at age 69. "Dutch, you don't look a day over 49," one man shouted.

His timing was perfect; his delivery flawless. His voice, not long ago so brittle it cracked at the wrong places and stumbled over familiar lines, was strong and sure. "This is the best place I've been all day," Reagan declared.

The crowd at the Sun City Center loved it. They were men and women in their 60s and 70s, retirees from the Midwest for the most part.

"I was really for Bush when I came to this rally," exclaimed Ruth Young.

73 and tanned to near perfection. "But Reagan impressed me. I think he's changed. He really impressed me with his youth."

Wherever Reagan has gone since his campaign moved South after his stunning win in the New Hampshire primary, the crowds have been large and enthusiastic.

And once again Reagan is the clear front-runner in the four southern states holding primaries this month, on Saturday South Carolina (GOP only) and on March 11, Florida, Alabama and Georgia.

Only 10 days ago, George Bush, former CIA director and ambassador, was riding high here and running surprisingly close to Reagan in a region once considered the former California governor's private preserve.

The South offered Bush a real opportunity to deal Reagan a crippling blow and eliminate at least one other GOP rival, John B. Connally. A four-state poll by Darden Associates for the Atlanta Journal found Bush leading Reagan by 42 to 32 percent. Former Texas governor Connally had 12 percent; Sen. Howard H. Baker Jr. (Tenn.) had 9.4 percent.

Reagan's victory in New Hampshire changed that. The only real question is how much.

In 1976, when Reagan narrowly lost to President Ford in the New Hampshire primary, his popularity dropped 18 points almost overnight in Florida. David Keene, then Reagan's southern coordinator and now Bush's national political director, says the attention the January Iowa precinct caucuses received will cut down the slide this year, but he estimates Bush has dropped at least 10 points.

"I suspect we're in second place in al of the states," Keene adds. "Reagan will probably win most of them by a country mile. Last week that country mile was a half a mile."

Florida, where almost four of every five registered Republicans are retirees from the Midwest or East, offers Bush the only real chance for an upset. "It's different than the rest of the South," says Keene. "People are conservative but they're not ideological conservatives. They're basically cautious Midwesterners."

But political professionals think Bush, who had been running neck-and-neck with Reagan in the state, has slipped badly. "There's a real chance [for Reagan] to carry all the congressional districts here," Reagan state chairman L. E. (Tommy) Thomas said last week.

Bush's campaign in Florida and throughout the South faces a problem. It concentrated early organization efforts in Florida and to a lesser extent Alabama, in effect conceding Georgia and South Carolina to Reagan. But after Bush's upset victory in the Iowa precinct caucuses, he skyrocketed in the polls and a strong showing in the South suddenly seemed possible.

Bush's standing in the region and his southern strategy, however, was built almost entirely on media and momentum. Most southerners actually knew very little about the candidate. "He's the only Republican who had gone up in the polls in a year. It was all momentum for him," pollster Claiborne Darden said. "Southerners seem much more interested in being with a winner than ideology this year."

When he lost by 2 to 1 in New Hampshire, the pressure built on Bush to show he could regain his momentum. He turned his immediate attention to the Massachusetts and Vermont primaries, to be held Tuesday, hoping to recover and cut down on time devoted to the South.

While Bush campaigned in New England Friday and Saturday, Reagan appeared before crowds of 1,800 in Sarasota, 5,000 in Bradenton, 1,100 in Sun City, 400 in Tampa, 800 in Savannah, Ga., 1,200 in Greenville, S.C., and 150 on a cold night in Columbia, S.C.

The South Carolina appearances were the most crucial. The state's primary on Saturday is the first and potentially the most interesting in the South. The campaign there is also the dirtiest, rife with charges and countercharges.

With no registration by party, no competing Democratic primary and an open invitation by GOP state chairman Dan Ross and Sen. Strom Thurmond for Democrats to participate in Republican voting, an unusually large and unpredictable turnout is expected.

There are signs that some Democrats will accept the invitation and possible throw a monkey wrench into the process. Belinda Friedmen, who attended both the 1972 and 1976 Democratic National Conventions as an alternate delegate, is one who intends to vote for Bush.

"I'm supporting President Carter," she says. "But I don't think he's going to need my vote for the nomination and if he loses in November I don't want Reagan or Connally to be president.

Just how much of this type support Bush will get is unpredictable. He floundered last Thursday, and his campaign has been enmeshed in a messy exchange of charges with Connally forces over an alleged vote-buying scheme that both sides accuse the other of instigating.

Lee Atwater, Reagan's South Carolina coordinator, claims the tide is running against Bush. "Bush's Big Mo got mowed down," he said. "The only thing he's got left is Massachusetts and all Massachusetts is is the most liberal state in the country. And I don't think that will go down very well here."

Ironically, Tennessee's Baker, the candidate with the strongest regional ties, made an early decision to concentrate his limited resources in Iowa and New Hampshire and is making only a token effort in the South -- much to the chagrin of his supporters.