COLUMBIA, S.C. -- An FBI sting operation promises to reveal not only a cozy relationship -- involving cards, women, cash and sometimes cocaine -- between legislators and lobbyists here, but also the weaknesses of a state government structure that was designed to support the once-powerful but fading cotton and textile establishment of the rural South.

With a Jaguar-driving, cigar-smoking legislator turned lobbyist as their front man, the FBI during the last legislative session videotaped alleged vote-buying in an investigation that is expected to result in the indictment of 15 or more state lawmakers and officials. As cameras secretly rolled, lobbyist Ron Cobb handed over packets of $100 bills in campaign contributions to state senators and representatives prepared to back a parimutuel betting bill.

"Let me give you a little gas for your tank," Cobb said as he made the payoffs, according to one legislator involved in the investigation.

In a state where financial disclosure and campaign finance regulation are among the weakest in the nation, the sting -- dubbed "Operation Broken Trust" by one newspaper -- already has provoked calls for reform and is expected to become a major factor in the November elections and next year's session of the legislature.

The FBI operation coincided with a separate series of state and federal criminal investigations involving a major public utility, the state Highway Department, the Highway Patrol, the state aeronautics commission and a junior state legislator who set up an "escort service" in his offices here in the capitol.

For South Carolina's ambitious governor, Carroll A. Campbell Jr. (R), the criminal investigations could either tarnish his steadily improving national reputation or provide an opportunity to begin shifting power from the legislative to the executive branch.

The power of the governor of South Carolina remains in many respects subordinate to that of the legislature, which controls, among other fiefdoms, the Highway Department, the Public Service Commission and the State Board of Education.

The domination of South Carolina government by a legislature controlled by rural interests first came under siege with the Supreme Court's one-man, one vote ruling in 1962. Court rulings in the wake of the Voting Rights Act of 1965 requiring adoption of single-member state House and Senate districts forced the break-up of county machines, and opened state politics to the influence of blacks, Republicans, independents and women. In the process, legislative leaders who once exercised near-absolute control over governors lost their leverage.

But while the power of the old guard was fractured by these developments, the structure of state government remained largely in place, with the allocation of exceptional constitutional authority to such figures as the chairmen of the House Ways and Means Committee and the Senate Finance Committee.

The Federal Bureau of Investigation sting operation has not yet produced any specific charges or indictments, but it has already had wide-ranging political consequences. A state press corps suddenly invigorated by the disclosures growing out of the Cobb sting operation has begun to reveal some of the worst internal workings of the legislative process here.

Cobb, 41, a Democrat who served two terms in the House from Greenville, was turned into an undercover operator after he was caught with cocaine, according to a number of reports here. For nearly a decade, Cobb's life had been in a nose dive. His first marriage ended in divorce in 1983, his construction firm failed in 1984, he came in third in the Democratic primary that year, and in 1988 his second wife shot and seriously injured herself as they argued on the telephone.

Cobb set up a lobbying firm in 1985 and moved into the Town House Hotel on Gervais Street here, holding court in the restaurant-bar and joining in the high-stakes poker game.

Once Cobb became a secret FBI operative, both his room at the Town House and his office in the AT&T Building next to the capitol reportedly were equipped with sound and videotape equipment, and he began to call legislators in to discuss campaign contributions and a pending bill to allow parimutuel betting. One of those calls was to state Sen. Theo W. Mitchell, the Democratic gubernatorial challenger to Campbell.

"He called me Thee," Mitchell recalled. " 'Thee, I have clients who want to make a contribution to your campaign,' he said. I said fine. He said, 'Old buddy, your old buddy is going to need some help on a little ole bill that's in your committee, that parimutuel betting bill.' I looked at him and I said, 'Ron,' I said, 'Ron, to the best of my knowledge, I can't accept a contribution that is tied to any legislation.' I said, 'That is wrong.' I said, 'You can't buy my vote, you know me better than that.' " Mitchell appears confident that any videotapes of the conversation will support his version of what occurred.

Cobb, in the meantime, has checked out of the Town House and disappeared into what is widely believed to be protective federal custody, as the federal grand jury prepares indictments, which are expected to be issued before the November election.

So far, at least 16 legislators have acknowledged that they have been either subpoenaed by the grand jury or contacted by FBI agents, including two who say they have been formally notified that they are targets in a criminal investigation. Five of the 16 are Republicans, a number roughly proportional to their share of the legislature.

South Carolina permits cash contributions to legislative campaigns and places no limits on the amounts candidates can receive. Although few of the senators and representatives in the investigation face serious election challenges this year, the defeat in the June Democratic primary of the House Ways and Means Committee chairman, who reportedly spent $56,000 in his failed bid, has sent a chill down the spines of all incumbents who say they feel constant pressure to maintain substantial campaign war chests.

When Cobb presented legislators, who are paid $10,000 a year and $8,500 in expenses, with the opportunity to pick up campaign cash, "it was like offering candy to a baby," state Rep. Ennis Fant (D), who took $1,000, told reporters.

In the long run, the current series of scandals is likely to accelerate the growth of suburban political power in this state. Not only are suburban politicians and constituencies the most concerned with reform issues, but the scandals provide the opportunity for the suburban, and generally Republican, legislators to challenge a structure of government designed to support rural interests that are declining in importance across South Carolina and much of the deep South.