With its pledge of new aid to Poland, the Soviet Union has put the western nations more firmly on the spot in what they are prepared to gamble to help salvage the shattered Polish economy.

The burden of decision falls heaviest on the United States as the leader of the West. Even before the Soviet Union announced Saturday its intention to defer Polish debts for five years and to provide emergency food supplies, U.S. officials conceded that one of the Reagan administration's most difficult choices could involve aid to Poland, where its foreign policy imperatives collide with its budget-cutting commitments.

"It is a massively complex problem," said one senior U.S. official.

For weeks the administration and other western governments have been debating behind the scenes whether to invest billions of dollars in new aid and credits in Poland. Each nation had its own arguments against making major new outlays, while conceding the need to support Poland's historic experiment in extracting a measure of "political pluralism" inside the Soviet bloc.

That foundering nation already owes the West $26 billion to $27 billion, depending on how the debts are calculated. "It is virtually in a state of international bankruptcy," said one U.S. official.

The secrecy of the communist system conceals the specifics of Poland's debt to the Soviet Union. The same secrecy could raise questions about how the Soviets perform in providing the new support promised to Poland's leaders during the Crimean meeting with Soviet President Leonid Brezhnev.

In any event, U.S. specialists agree, the Soviet Union can bring Poland to its knees any time it chooses through its economic leverage. Without sending one soldier over the Polish border, American analysts point out, the Soviet Union can "turn off the spigot" of economic support, either subtly or openly, "and blame the West for the collapse of Poland."

Support for Warsaw thus is a gamble the West cannot avoid, and must now take, diplomats in the State Department and in other western foreign ministries are telling their political leaders. To reject it, foreign policy professionals insist, would be to abandon for years any prospect for loosening up the communist system.

Therefore, they argue, aid to Poland should be given in the largest strategic sense as a matter involving western security, transcending normal foreign aid and budgetary limitations.

Here and in other western capitals, specialists have been exploring a possible multibillion-dollar program of international aid and credits for Poland, possibly channeled and coordinated through the International Monetary Fund.

These discussions have been clouded, however, not only by suspicions about Soviet intentions and debates about the survivability of Poland's economy, but also by internal western disagreement over the forms of aid each nation might contribute.

The advocates of new aid maintain that East and West, while divided over political objectives in Poland, have "a joint interest in keeping the Polish economy afloat." The new pledges of Soviet aid to Poland over the weekend undoubtedly will be cited to strengthen that argument.

Inevitably, however, opponents of major new support for Warsaw will cite in turn the emphasis in Saturday's Polish-Soviet communique on strengthening the "guiding role" of Poland's Communist Party and combatting "subversive actions of the forces hostile to socialism" in Poland.

Inside the Reagan administration there is strong opposition in the budget-conscious top echelon to any major new cash outlays for Poland.

As American officials describe the attitudes that emerged in recent talks with its European allies, the United States proposed contributing primarily food to Poland, while the Europeans pressed for contributions largely in cash and credits. The result was a standoff, although American officials express hope that a compromise can be reached.

A senior U.S. official said, "We wish to focus basically on food credits, agricultural reform," and remodeling the price structure of the Polish economy. The Europeans, he said, "would focus on direct balance-of-payment support and industrial credits."

What Poland needs most urgently, he said, are "incentives for private farming" to meet its critical food shortages.

The U.S. formula would draw heavily on American stocks of surplus food -- obviously a far less burdensome outlay for the Reagan administration than cash.

Beyond what any individual nation may contribute to Poland, the major hope for salvaging its economy turns on bringing Poland into the International Monetary Fund.

An IMF-organized program would not only help sustain Poland, but would lend credibility to that nation's prospects for repaying its debts, and would help assure Poland's reform movement that an independent international agency would be monitoring the performance of its communist government.

To draw credits from the IMF, a nation is required to meet explicit conditions for the performance of its economy. The Soviet Union traditionally opposes such penetration of the Soviet bloc, although Romania is an IMF member.

Poland, nevertheless, has agreed, State Department officials said, to a proposal by the major western creditor nations to permit an IMF "observer" to attend the next meeting of Polish representatives and a 16-nation western task force on rescheduling Polish debts.

This group is scheduled to meet for three days starting Sept. 9, either in Warsaw or in Paris. The core countries in this group are the United States, Great Britain, France, West Germany and Austria. Special ambassador Charles F. Meissner will represent the United States at the meeting.

The Solidarity movement, which claims 10 million members out of Poland's population of 35 million, is scheduled to hold the first stage of its national congress Sept. 5-6. That conference and a subsequent session in early October are expected to show whether Poland can achieve enough national equilibrium to survive its experiment in modified communist rule.