Steadily declining U.S. agricultural exports have put the Reagan administration under new pressure to change its hard line on aid to financially stressed farmers.

The latest farm-state concerns were sparked when the U.S. Department of Agriculture last month made a $1 billion downward revision in its forecast for 1985 farm exports to $34.5 billion, down from last year's sales of $38 billion.

The March revision was the second since December, when USDA analysts were projecting $36.5 billion in sales for the new year. The new figure compares with the record high of $43.8 billion in 1981.

Within days of the revision, a delegation of farm organizations met with top White House officials to urge more federal export aid. They warned that President Reagan's 1985 farm bill proposals were doomed without a stronger commitment to export promotion.

And last week, more than a dozen House farm-state Republicans, voicing fears of further deterioration in exports, broke ranks with Reagan and said they will develop their own farm bill proposals aimed at promoting overseas sales.

With the U.S. farm economy buffeted by low prices, high interest rates and declining land values, the export picture takes on additional significance in lifting American producers from the doldrums. About 40 percent of U.S. food production goes overseas; the USDA says that by the year 2000, half of U.S. production will be exported.

The newest forecasts of declining exports, which most analysts agree are chiefly a result of the strong dollar that makes U.S. grains less competitive on the world market, also have raised the specter of enormous new U.S. surplus accumulations, particularly of wheat, and new costs to the government.

"It looks very grim," said Carl Schwenson of the National Association of Wheat Growers. "We are looking at a 1.5 billion-bushel wheat carryover at the end of May -- and that would be a historic high -- with our share of the world market 44 percent less than it was last year.

"Most people believe the USDA will have to lower its export forecasts even more," he said. "Aside from our traditional markets, not much grain is moving."

The export picture was clouded even further with a federal court ruling that cargo-preference law applies to grain shipped under the federal blended credit program, which is designed to promote sales. The decision means that half of government cargoes must move on U.S.-flag vessels.

USDA's response was to suspend the $536 million program and regroup. Agriculture Secretary John R. Block announced last week that the administration will support farm-state legislators' efforts to enact an exemption for blended credit grain sales from cargo-preference law.

In an interview last week, Daniel G. Amstutz, undersecretary for international affairs and commodity programs, declined to describe the export picture as "bleak," but he agreed that it is cause for major concern.

"I wish we would call these forecasts 'guesstimates,' because that is what they are," Amstutz said. "The big variable continues to be the vagaries of the weather, which can have a significant effect on our forecasts.

"But on a global basis, with current national policies and assuming continuation of weather of the last several years, world food production will increase faster than consumption and world trade. The size of the pie is getting smaller for all who want a piece of it," he said.

Amstutz also refused to blame the strong dollar for the U.S. export drop. "No question that plays a role," he said, "and my sense is that if the dollar lost half of what it has gained since 1981, our exports would be up about 10 percent this year.

"But there are other factors at work. It relates to the incentives for farm production around the world, juxtaposed on the reality that the world market is not growing as fast as it was in the last half of the decade of the 1970s . . . . We will soon be in a position in which we need only 50 percent of our agricultural resources to feed ourselves."

That point -- and the strong reliance that U.S. farmers place on overseas sales -- looms heavily over the congressional debate on a new farm bill. Much of this country's excess production capacity, along with the heavier debt it meant for farmers, was stimulated during the export boom of the 1970s. Government officials urged farmers to expand to meet the demand, which many thought would continue indefinitely.

But the lingering global recession and increased farm production by U.S. competitors in Europe and the Southern Hemisphere, in tandem with the strength of the dollar, have sent the U.S. export curve downward each year since 1981.

Administration critics charge that Reagan's farm bill proposals, which have almost no support on Capitol Hill, will put tens of thousands of farmers out of business in his drive to cut farm program costs and make U.S. prices more competitive abroad. Rep. Pat Roberts (R-Kan.) said the Reagan plan would send farmers marching through a "Valley of Death."

Block, Amstutz and others contend that the U.S. price support programs, which set a floor under grain, rice and cotton prices, put a protective umbrella over foreign farm production and make U.S. products less salable.

"We hope to address the supply-demand imbalance through our farm bill," Amstutz said. "The farm bill goes in tandem with what we are trying to achieve on overcoming trade barriers and discriminatory practices around the world. The first among the discriminatory practices is the subsidy.

"We're also saying with our farm bill that we're probably the biggest culprit with our agricultural production incentives. We have barriers and subsidies, also. It's not just a one-way street."

Despite the congressional coolness, Amstutz said, the administration intends to continue to press for reduced farm price supports as a way of forcing other major producing countries to cut their own subsidization of agriculture.

He also indicated strong opposition to congressional Republicans' proposals to tailor U.S. farm supports to world price levels, with provisions for paying direct subsidies to farmers to make up the difference between their sales price and their production costs.