(FORTUNE Magazine) – Treasury Secretary James Baker envisions a three-pronged attack on the problems of 15 debt-laden developing countries. On one front he wants the debtor countries to make structural changes in their economies that will increase growth, produce sound balance-of-payments positions, and cut inflation. The countries are supposed to strengthen their private sectors and reduce budget deficits, put in supply-side programs such as tax reductions, liberalize trade, and promote foreign investment. Such policies, Baker says, will build confidence, encouraging domestic savings and stopping the flight of capital. If the debtors buckle down to those prescribed changes, lenders open Fronts 2 and 3: commercial banks around the world grant the countries about $20 billion of new loans over the next three years; and the so-called multilateral development banks -- the World Bank, the Inter-American Development Bank, and others -- supply another $20 billion. Lending would continue case by case, with each country's loans negotiated separately. Baker wants to see the World Bank play a major role in making good things happen. In recent years it has begun making what it calls ''policy-based loans,'' which aim at Baker-like structural changes. For example, the Bank finances certain imports for countries willing to knock down trade barriers that would have kept those imports out. In almost all its loans it operates with carrot and stick, doling out money only as borrowers make headway. Baker wants the Bank to prod the 15 countries. But behind the facade of this plan are endless complications that may scuttle it: Baker's structural changes add up to political dynamite and are impossible to put into effect quickly, if at all. The Reagan Administration has campaigned for such changes before and has got so little that Baker is making another pass at the well. Lining up the banks will be an organizational nightmare. For the plan even to get started, some 700 banks in 50 nations have to commit to lend $20 billion, providing those structural changes get made. In percentage terms the money asked is not big, amounting to only 7.3% of the banks' outstanding loans to the 15 countries, or an annual increase of about 2.4%. The U.S. money center banks can see a compelling reason to put that up: the investment promises to keep interest payments on existing loans flowing in. Also, as Baker has said, the big banks ''have an ox in the ditch.'' They are looking for any way to get that fellow out. But U.S. regional banks have reason to balk. Many have written down their foreign loans by a lot or sold loans to eliminate their exposure to a given country. Suppose a regional has largely written down its Brazilian loans and is out of Argentina entirely. Is it now to get back into Argentina by joining in the $20-billion commitment? Whoa. As a banker bitingly says, ''This is not a new-business effort.'' Yet the cooperation of the regionals is essential to Baker's plan because the money center banks aren't ready to sign on by themselves. Were they to do that and be called on for new loans, part of the money would flow back to the regionals as interest on old loans. The World Bank is an imponderable: conservative, methodical, bureaucratic, a big borrower in the financial markets and therefore superprotective of its credit rating (AAA). In its 1985 fiscal year, a period of desperate need for many countries, it did not commit all the money it had expected to, partly because so few countries met its standards of creditworthiness. In recent congressional hearings, Baker has conceded that his plan will be difficult to execute and uncertain in results. But, he says, ''we do not see any alternative to an approach of this nature.'' In other words, count this as a last-ditch effort.

CHART: COUNTRY FOREIGN DEBT 1985 INTEREST DEBT OWED estimated % TO U.S. BANKS in billions in billions of 1985 GNP in billions ^ Brazil $103.5 $11.8 5.8% $23.8 Mexico 97.7 10.0 6.3 25.8 Argentina 50.8 5.1 7.9 8.1 Venezuela 32.6 4.1 8.1 10.6 Philippines 27.4 2.1 6.2 5.5 Chile 21.9 2.1 12.9 6.6 Yugoslavia 20.0 1.7 3.6 2.4 Nigeria 18.0 1.8 1.9 1.5 Morocco 14.4 1.0 8.2 0.9 Peru 13.9 1.3 10.8 2.1 Colombia 13.9 1.3 3.3 2.6 Ecuador 7.9 0.7 6.0 2.2 Ivory Coast 6.3 0.6 8.7 0.5 Uruguay 4.9 0.5 9.8 1.0 Bolivia 4.2 0.4 10.0 0.2 TOTAL $437.4 $44.5 Average 7.3% $93.8 A BAKER'S DOZEN PLUS TWO For U.S. banks Latin America is the zone of pain. But Baker Swanted support for is plan from foreign banks with problem loans concentrated elsewhere, so his lis of troubled debtors has geographical spread. Variable interest Srates apply to ore than half the debt, which means the interest burden can swing sharply. Interest figures show what the countries owed in S1985 -- but som , such as Peru and Bolivia, didn't pay in full.