Washington — Which approach is most effective in stemming the drug tide: gentle persuasion - or ultimatums? Can you stop other countries growing coca bushes (cocaine) and opium poppies (heroin) by appealing to them - or by threatening to cut off foreign aid unless they do?
World supplies of heroin, cocaine, and marijuana are so huge that the US answer is to try to get tougher. A new law passed in mid-November would cut off United States aid to nations such as Colombia, Peru, and Bolivia (coca bushes) and Pakistan and Thailand (opium poppies) if they fail to take certain annual steps decided on in advance by the US president.
In effect, Congress is still prepared to let the president make the key decisions.
It has rejected the clenched-fist approach of Rep. Charles Rangel (D) of New York and others which says, ''Reduce your supplies by 10 percent a year or else. . . .''
Instead, a majority in both the Senate and House has tried to steer a middle course. As explained by House Foreign Affairs Committee sources, the law would:
* Make the president decide the maximum amount by which each producing country could be expected to reduce supplies each year.
* Make him report to Congress each February on what each country had done, and whether it had met the maximum requirement.
If progress was thought unsatisfactory, Congress could withhold aid funds.
The State Department argues that some countries such as Pakistan and Thailand cannot make rapid progress because they lack resources and face rebellious tribes and guerrillas in growing areas.
Besides, the department adds, the US has other foreign-policy interests to balance: Colombian efforts to find peace in Central America, Thai treatment of refugees from Vietnam, Kampuchea, and Laos, and Pakistan's role as a front-line state against the Soviets in Afghanistan.
The department's dilemma is to draft annual reports for the president that keep foreign aid continuing, but also to insure at least some visible reduction.
The new law is weaker than an earlier amendment from Sen. Paula Hawkins (R) of Florida. She would have ordered producer nations to eliminate supplies completely in just a few years.
What might its effects be?
''The new law increases pressure on the executive to come up with more facts and figures and answers,'' says a House committee source. ''It is also another piece of pressure on the producing countries, and they are aware of it.''
When all is said and done, however, a more effective way of reducing coca leaf and poppy cultivation around the world may well be something over which Congress has no control at all: The tragic rapidity with which heroin and cocaine addiction rates are soaring in the growing countries themselves.