The chairmanship of the Senate Foreign Relations Committee may be decided by the power of tobacco, a golden leaf that holds extraordinary sway over American politics.

Sen. Jesse Helms (R-N.C.), who represents the nation's biggest tobacco state, reportedly is under pressure from fellow conservatives to give up his chairmanship of the Agriculture Committee to take over Foreign Relations.

Helms has not commented publicly on the possibility of a move. But during his recent reelection campaign he stressed repeatedly that he intended to stay on as chairman of the Agriculture Committee to defend his state's tobacco farmers.

Helms warned that if he was not returned to Washington, the agriculture panel's chairmanship would go to Sen. Richard G. Lugar (R-Ind.), whom he called a "threat" to the tobacco price-support program.

However, that was before the chairman of Foreign Relations Committee, Sen. Charles H. Percy (R-Ill.), was defeated. Helms is next in line to succeed him.

Lugar, for his part, is more interested in becoming Senate majority leader than in heading the Agriculture Committee. But in a speech Monday to a peanut industry group, he said that if he became agriculture chairman he would take the lead in revamping farm programs -- a clear signal that he would focus on tobacco, just as he focused on the peanut price-support program in a 1981 assault on farm supports.

With Helms gone from the agriculture chairmanship, there would be no remaining heavyweight defender of tobacco. The ranking Democrat and other principal tobacco champion on the committee, Sen. Walter D. Huddleston (D-Ky.), lost his reelection bid last week.

All of this may be somewhat academic because, even with Helms remaining as Agriculture Committee chairman, virtually all industry officials and many tobacco growers think that the complicated program may be beyond saving.

Tobacco got into its first tough legislative scrape in the farm bill debate in 1981. Critics charged that it represents an unwarranted government subsidy of a product clearly identified as a health threat. Tobacco-state legislators promised to reshape the program to erase any hint of subsidy.

The result, in 1982, was adoption of a price-support scheme that required farmers to pay the cost of their program, at the rate of a 7 cents per pound levy on tobacco they grow.

Helms and others promised that flue-cured and burley tobacco, the main ingredients of cigarettes, would no longer be a problem for a Congress growing increasingly uneasy about all farm-support programs.

But a year later, they were back asking for a one-year freeze on tobacco-price supports, a move aimed at keeping American leaf competitive with the growing influx of imports and avoiding the possibility of more unsold U.S. tobacco going on loan into the cooperative warehouses.

But that didn't work, and the tobacco people came back last year seeking an extension of the price-support freeze through 1985 and changes in other facets of the program. There are new signs that these changes have not been the salvation that tobacco farmers expected, and there is a move for more legislative fixes in 1985.

Under the current program, farmers of flue-cured tobacco are facing a possible levy of 15 cents a pound next year to help pay for costs of the program and the possibility of more cuts in their poundage quotas -- steps that would make tobacco growing a losing proposition for most of them.

Basically, the program works this way: Tobacco is grown under a system of controls that include acreage allotments and poundage quotas. Farmers who are unable to sell their tobacco may post the leaf as collateral and receive a loan at the price-support rate (now $1.69 a pound). When the leaf is sold from the cooperative warehouse, the farmer repays the loan and takes any remaining profit.

But about four of every five tobacco allotments are owned by nonfarmers, who rent their growing "franchises" to farmers. Lease rates in most major growing areas are at least 50 cents a pound and in some cases as high as 80 cents -- on top of the $1.11 that the Agriculture Department says it costs to raise an average pound of tobacco.

Legislative efforts to eliminate the allotment system -- at least to get nonfarmers out of the leasing business -- have been slowed by the owners' political clout, although Congress has required that some allotments be phased out in 1987.

"This program will self-destruct without some change," an industry official said yesterday. "At $1.11 a pound, plus the likely 15-cents levy, plus the 50 to 80-cent lease fees, there's no way a farmer can make it when the support price is $1.69. It's that simple."