John B. Connally's Southern ambush of Ronald Reagan's presidential bandwagon is being planned not for the highly publicized March 11 florida primary, but for three days earlier in a South Carolina election that has been overlooked nationally.

Working quietly, Connally is planning a media extravaganza costing nearly $400,000 (out of the $600,000 spending limit for the primary election). The goal: to exploit a potentially grievous Reagan blunder in permitting South Carolina's Republican delegates to be picked by primary instead of convention.

For Connally, what happens in this state's first Republican presidential primary could prove to be his making -- or breaking. If front-runner Reagan heads south after winning opening rounds in the Iowa caucuses and the New Hamphire and Massachusetts primary elections, Connally must hurst him in Dixie. If he fails to do that, Connally will be finished and Reagan all but nominated.

Tackling Reagan in Florida never did look all that promising. Nor do primary elections, also March 11, in Alabama and Georgia offer anything like the same impact as South Carolina on March 8. This home state of Sen. Strom Thurmond has the Deep South's strongest Republican Party.

Thus, when the South Carolina party decided in August to hold a presidential primary and tacked on a $1,500 entrance fee to keep out adventurers, Connally was handed a perfect setting to ambush Reagan.

With no competing Democratic primary and no party registration so that every voter can vote, the turnout could be as high as 100,000 (compared with 35,000 in the 1974 Republican governor's primary). Connally could draw well from conservative Democrats -- better tan Reagan. Yet, ironically, Reagan helped set the primary stage himself.

Reagan's reasoning was based on fear of the immense party infuence of Thurmond and former governor James B. Edwards. Reagan operatives worried that, in precinct and district caucuses followed by a party convention, Thurmond and Edwards might swing delegates to Connally. Their part power would then undercut Reagan's dedicated precinct workers.

With both Thurmond and Edwards refusing to make early endorsements of Reagan, the simple way to short-circuit their influence would be a primary election. That exactly conicided with the hopes of South Carolina party officials. They figured that the expected stream of Democrats into the Republican primary could generate major political dividends.

Reagan's operatives failed to foresee that Connally would seize on the new winner-take-all primary. Reagan has would up risking a dramatic statewide loss to Connally with major national overtones merely to protect himself against a no-win, no-loss verdict, for him the worst conceivable result of a state convention.

Edwards, the most emotionally pro-Reagan Southern Republican in the 1976 Ford-Reagan shoot-out, is shifting toward Connally. Although he resists Connally's request for a public endorsement and told us "I haven't made up my mind yet," intimates are betting he will end up campaigning for Connally.

Thurmond has also stayed out. But Thurmond's wife is known to favor Connally over Reagan. Moreover, Thurmond told one political ally here before Sen. Edward Kennedy entered the Democratic race that "if Teddy gets the Democratic nomination, Connally has to be our man."

Charles Black, Reagan's national field director, may have inadvertently pushed Edwards and Thurmond toward Connally by tapping freshman Rep. Carroll Campbell Jr. as state campaign chairman. Although he is a bright young Republican comer, Campbell has become a red flag to Thurmond and Edwards.

Edward's grievance: when Campbell lost as candidate for lieutenant governor (while Edwards was winning) in 1974, he played a cozy game with Charles (pug) Ravenel, briefly Edwards' Democratic opponent until ruled off the ballot. Edwards never forgot.

Campbell's first act after being named Reagan's chairman did not conciliate Thurmond. On a state tour, Campbell dropped hints that his new eminence made him the state's top Republican.

Connally plans 10 full days in South Carolina, including stumping in such small towns as Aiken and Anderson. That effort is essential against Reagan, the South's most popular Republican. But the task of protecting that reputation is assuming a larger dimension today for a front-runner who cannot reveal Southern weakness without undermining the foundation of his national party strength.