If further evidence of the failure of the Reagan Administration's third-world debt policy were needed, the $3.5 billion loan to Mexico is it.

In Mexico's view, since 1982 it has done all that foreign creditors have asked. It has taken on more debt, reduced wages, slashed the Government's operating budget, started selling state enterprises, devalued the currency, reduced tariffs, eased restrictions on foreign investment and encouraged non-oil exports to offset the decline in oil revenue.So why the need for emergency financing? Just a gesture of support to encourage further reforms in the face of growing popular opposition to reduced living standards? After all, $3.5 billion is a pretty big gesture for an Administration that has pledged never to substitute taxpayers' funds for our commercial banks' troubled third world loans. The size of the credit line and haste of the negotiations suggest that Mexico was again in financial difficulty.

The source of the trouble is easy to find. This year, the Mexican Government did what any incumbent does to retain office: It tried to make the economy look good before Mexico's Presidential election. It slowed inflation and increased the availability of consumer goods by negotiating a wage-and-price pact with workers and manufacturers and by holding the peso exchange rate steady while lowering tariffs and licensing requirements to increase the inflow of foreign goods. A 60 percent increase in imports this year not only helped satisfy consumer demand but also held down domestic prices.

But the successful political strategy wreaked havoc with Mexico's balance of payments, already hit by a further drop in oil prices.

The trade surplus shrank, leading to a widespread conviction that Mexico's presidential transition will be accompanied by another major devaluation of the peso. Capital flight has surged. Knowledgeable bankers say foreign exchange reserves that earlier this year stood at more than $16 billion had fallen to less than $10 billion by late September, and the outflow has apparently accelerated since then.

Mexico's dilemma is not unique. Our debt policy of the last six years has left all the major debtor nations in a trap. They can run a successful external balance of payments policy or can try to satisfy their growing population's demand for a better life. But they cannot do both.

There is growing acceptance in the international financial community that the only way out of the debt trap is debt reduction.

The most serious obstacle to a more realistic debt policy is the United States Treasury, which refuses even to discuss any change of course.

One would hope that the next administration, Democratic or Republican, will seek real solutions to a problem that for six years has weakened the global economy, eroded the integrity of the financial system, victimized poor people in poor countries and threatened vital economic and strategic interests of the United States.