The House of Representatives has sent South Africa a message: You can no longer count on us to play the role of NATO -- No Action, Talk Only.

The vehicle for the message was Wednesday's approval of sweeping sanctions, including complete divestment, an embargo on all trade and the severing of all economic ties with South Africa. The ease with which the measure cleared the House, surprising even its backers, must have shocked Pretoria.

But there is ground for thinking that the House action might have been too successful. That is, the sanctions are so draconian that they will have trouble gaining Senate approval or clearing the White House.

3 A president who has allowed American oil companies to continue doing business with Libya's leader, whom he clearly detests, would almost certainly veto sanctions as radical as these against a country he sees as a bastion of anticommunism -- especially when he can do so while appearing reasonable.

Instead of basking in the glow of their astonishing victory, congressional opponents of apartheid might devote the days ahead to some serious reflection. I commend to their attention a piece in the June 14 issue of The Economist.

The British magazine begins by pointing out the "dismal record" of sanctions. "They are a legitimate way of expressing distaste or despair, but they rarely change the things that cause those feelings. They can all too often be evaded. They sometimes stiffen the backs of the sanctioned and spawn dissent among the sanctioners."

This is no argument against sanctions, which remain "one of the few extensions of diplomacy available to governments short of war," but only a reminder to think soberly about what sanctions can and cannot do.

They can shake a recalcitrant government out of its complacency. But whatever South Africa is, it is hardly complacent. They can register the outrage of the outside world, but that outrage can hardly be in doubt.

But can they force change? Yes, says The Economist, if they are properly thought out and logically applied. It proposes a checklist to avoid falling into the traps of impracticality, counterproductiveness and self-gratification:

1. Does it matter how the black majority comes to power? It does. "The West would like to see a relatively bloodless transition to a democracy that will stay a democracy, preserve human rights and a free-market economy . . . and keep South Africa within the community of western ideas."

2. Do we have a clear picture of what we would want sanctions to encourage? We don't, and we must. The Eminent Persons Group from the Commonwealth may have failed in its effort to move South Africa, but its ideas were specific: "Legalize the African National Congress in return for a cease-fire. Free Nelson Mandela and other political prisoners. Get all parties around a table to try to negotiate a new constitution that will enfranchise blacks." To work, sanctions must have clear objectives.

3. Whose minds would we hope to change through sanctions? Not the reactionary Afrikaner right (a fifth of the white vote), whose minds won't be changed. Not the liberals of the Progressive Federal Party (another 20 percent), who need no persuading. "The target must be the main body of the ruling National party, whose mandate President Botha needs if he is to face down the far right."

4. Can a credible set of new sanctions be assembled? The measure passed by the House, for all its signal-sending significance, is simply not credible; South Africa cannot believe it will be implemented. And even if it were, it "might do a lot for change in southern Africa, but not for the sort of change the West wants."

What then? Specific, credible, enforceable measures -- such as a ban on all military contacts and a moratorium on passenger air travel between South Africa and the West -- in support of such specific South African actions as the release of prisoners, the unbanning of the ANC and serious negotiations with legitimate black leaders.

The Economist's proposals might not trigger the feel-good applause that followed Wednesday's House action, but they strike me as a reasonable, effective and veto-proof roadmap for real change.