Contradictions that give Republican presidential fortunes in the Southland their exotic quality were on lurid display when applause roared out for John B. Connally's luncheon speech here Saturday during the party's southern regional conference, far out-thundering Ronald Reagan's reception the previous day.

Listening to the roar of the crowd (only partially due to table-packing by Connally agents), one Republican leader murmured: "Magnificent, really magnificent, but Big John can't take it out of the room with him."

Only the most ardent Connally supporter would dispute that truism about Connally's failing campaign, even here in his own back yard where the tough-guy Texan has pinned his hopes. With Reagan's southern base displaying cracks from the Jan. 21 Iowa tremor induced by Geroge Bush, Connally must show that he, not nice-guy George, will inherit any Reaganites who decide Reagan cannot make it after all.

Just how large an order that is proving to be for the dynamic Texan is now becoming clear in South Carolina, a "must" state for Connally and the state that dramatizes the fascinating contradictions of the Republican presidential race.

Weeks ago Connally nailed down the backing of Sen. Strom Thurmond, the party's southern eminence, and former Republican governor James B. Edwards for the March 8 South Carolina primary. He then abandoned federal matching funds to give him unlimited spending latitude in a state that never before has had the luxury of a Republican presidential primary election.

It seemed just possible that he might catch up to Reagan's strong grass-roots support in South Carolina. But if so, the prospect dimmed on Monday when Harry Dent, Thurmond's longtime political adviser, led a Republican platoon from one end of the state to the other trumpeting for George Bush. Not only is Dent one of the most formidable southern Republican power-brokers (who with Thurmond saved the South for Richard Nixon in 1968) but he also is advertising his new allegiance to Bush with a seductive line. "I tried to be for John Connally," says Dent, "but by now everybody knows that he just can't win." Dent thinks Bush will steal votes from Connally and ensure an anemic tally for Sen. Howard Baker in the South Carolina election.

Contradictions abound in the alliances that have formed in support of Reagan, Connally and Bush, a fact that tends to obliterate minor ideological distinctions. On Reagan's one-day campaign swing through South Carolina on his way here last weekend, for example, Reagan operatives truthfully bragged that many of Reagan's presidential campaign aides cut their political teeth for Thurmond or Edwards but, like Harry Dent, declined to follow them to the Connally standard.

Likewise, freshman Rep. Carroll Campbell is Reagan's statewide campaign chairman while Richard Greer, who ran Cambell's 1978 congressional campaign, has signed up with Bush -- and Dent -- as political director of the Bush campaign.

This crazy-quilt pattern that cuts across old loyalties to fashion new alliances shows the fluid nature of 1980 Republican presidential politics, southern-style. It hints that if the vulnerability Reagan showed in Iowa continues,nothing will prevent Reaganites from moving into the camp of a perceived winner: George Bush.

What is questionable is whether Reagan remains vulnerable for the Feb. 26 New Hampshire primary or recovers his old appeal. Reagan told us on his swing through here last week that his Iowa defeat resulted from "complacency" that has now been rooted out.

But his campaign performance has raised other questions among the practical Republicans who flocked to this regional conference. One complained about his appearance on television ("terrible," he told us); another criticized his "imperialistic" attitudes. These vulnerabilities may be cured in New Hampshire if, as Reagan implies, the real problem in Iowa was in fact only "complacency," not some chronic or incurable political malaise.

If the latter, Bush is Reagan's legatee. As a longtime Alabama party leader told us here, "Bush is basically already eating up Connally's lunch." If Reagan stumbles in New England as he did in Iowa, Bush will dip into Reagan's lunch, too.