The failure of the new export-oriented federal farm law to produce a quick turnaround in slumping U.S. agricultural sales abroad is putting political pressure on the Reagan administration to take action.

The hue and cry is being led by Senate Majority Leader Robert J. Dole (R-Kan.), a reelection candidate in a major wheat state, who has warned that Congress may act by fall if the administration does not pursue exports more aggressively.

Joining Dole are farm groups, grain-trading firms and other legislators who contend that the Reagan administration has not made adequate use of various approaches outlined in the 1985 farm bill for juicing up exports.

Despite frequent warnings by agricultural economists and Agriculture Department officials that the law would not produce instant results, the deteriorating trade picture has created impatience and uncertainty about the administration's handling of the programs.

Although the lower prices spurred by the new farm law appear to be stimulating advance purchases of wheat, cotton and rice for next year, most economists continue to see bleak prospects for major market expansion.

"Any real recovery of export tonnage is now expected to take three to four years. . . and it may take 10 to 15 years to get back to peak levels of 1980-1981," economist John A. Schnittker wrote in a recent issue of Choices, a journal of the American Agricultural Economics Association.

The farm-country frustration comes in part from steadily eroding exports, which USDA predicts will fall to $27.5 billion this year from a $44 billion high in 1981 -- in part from the lower prices and in part from the first monthly farm trade deficit in 30 years, when imports exceeded exports in May.

On top of this discontent is the political equation, with a number of farm-state Republicans on the endangered list and with GOP control of the Senate hanging in the balance in the November election.

"I've already told Secretary Richard E. Lyng to consider the politics of this," said Sen. John Melcher (D-Mont.), one of the administration's strongest critics on export policy. "We have got to blunt the fall of prices this summer. I've urged them to do everything possible and not sit around and wait for the big sales that are not going to happen. If this continues through the fall, it might have an effect on the November elections.

"I've said they have to reach the bottom point this year," Melcher added. "There is no give after that. This is the year they've got to come back up economically and politically. Farmers and agribusinesses can't survive much longer than that."

Rep. Edward L. Madigan (R-Ill.), whose Corn Belt district is a major corn and soybean exporter, said that while he does not expect electoral fallout over exports in November, he does expect the administration to step up its efforts to move farm products abroad.

"Everybody would have hoped for more than has happened so far, but maybe it's too early," he said. "I think the administration will do more . . . we're going in the direction of more subsidization. We're not going to get out of this without spending more money."

Rep. Thomas A. Daschle (D-S.D.), running for a GOP-held Senate seat, said he expects the administration "to give us an 'October surprise,' but probably in September," to mollify farmers with an income sweetener. "But I don't care whether you pay other countries to take our grain, we have to realize that it is a whole different market out there."

Added Rep. Dan Glickman (D-Kan.): "I suspect after the elections there is going to be some fallout. Low grain prices and the huge budget exposure of the farm programs will force us to look at more aggressive supply management approaches . . . . There is a growing recognition that without some additional push on supply management, the administration focus on exports as a key point of its farm program will fail. Lower prices are where the fallout is going to be."

Administration policy-makers are reviewing possible moves to stimulate exports, including the possibility of extending subsidized grain sales to the Soviet Union. Dole said he "detected some willingness" by the administration to do this, partially as a stimulus for a second Reagan-Gorbachev summit meeting this year.

But USDA's chief export overseer, undersecretary for international affairs and commodity programs Daniel G. Amstutz, said in an interview last week that he envisioned no change in U.S. trading policy toward the Soviets, who have consistently argued that as a major U.S. customer they should get special treatment.

"I foresee no change in our policies and no change in the criteria for the export enhancement subsidy program. The Soviet Union is the only country for which we earmark supplies; they have an assurance of supply from us . . . . I am confident this country will fulfill its long-term agreements with the Soviets," he said.

Amstutz acknowledged the growing political pressure on the administration to make dramatic moves on the export front. He insisted, however, that "we have not been slow in implementing the export provisions of the farm bill."

"We knew from comments we heard after the bill was signed that if we didn't see an immediate response on exports, the pressure would start building. We've tried to be forthright, saying one could not expect an immediate response. There is a lag effect that is just not scientific," Amstutz said.

"Part of the frustration out there is that the market is not as large as it once was. The market has contracted, not just the U.S. share of it. And there is no short-term way to make the market grow," he said.