A nightmare haunts the upper reaches of the Japanese establishment: that the political influence of this nation's cattle-raisers and citrus-growers will provoke a protectionist reaction by the U.S. Congress, with ruinous consequences for the U.S.-Japanese alliance and the Western economy.

That nightmare was sufficiently vivid, even without explicit warnings this week from American congressmen junketing through the Orient following the U.S. elections. Thanks to dependence of the ruling Liberal Democratic party (LDP) on the small but strategically placed farm vote, there is not chance of lifting all quotas on fruit and beef. That political fact of life, though trivial in broad economic terms, could unlock the floodgates of trade protection.

Ironically, relations between the two countries are at their warmest since before the "Nixon shocks" of 1971 (President Nixon's unexpected China initiative and import-tax imposition). Today's doubts in Tokyo concern not President Carter's good intentions but his mastery over events.

"President Carter is much more of a free trader than President Nixon," Nobuhiko Ushiba> who as minister for external economic affairs is the counterpart of U.S. trade negotiator Robert Strauss. "But can Carter do as well as Nixon in controlling the Congress?"

Ushiba and other Japanese leaders, accustomed to orderly decision-making, are bewildered - and a little frightened - by fragmentation of power in Washington. Apart from Carter's weakness in Congress, they yearn for the good old days when they could concentrate on Wilbur Mills, in assurance that he could guarantee the outcome of trade legislation.

Retired Sen. Mike Mansfield, who has emerged as probably the most widely respected and popular in U.S. ambassador to Japan, has tried to explain the new congressional reality to Japanese officials. Symbolically, he has placed photographs of the joint congressional leadership on his office walls. "I did this not because there are my old friends," Mansfield told us, "but to show that it is not enough only to talk to the President and the secretary of state."

When hordes of congressmen turned up here Nov. 14 on their post-election migrations, Mansfield urged them to speak frankly to Japanese Diet members. They did just that. Three House Ways and Means Committee members - Reps. Sam Gibbons (D-Fla.), James Jones (D-Okla.) and William Frenzel (R-Minn.) - bluntly warned that the protectionist tiger will be let out of its cage unless Japan does more to ease U.S. imports. Kiichi Miyazawa, director of economic planning, listened in obvious displeased.

Gibbons stressed the dire consequences of not removing trading barriers against the oranges of his Florida constituents, which sounds like threatening World War III because of a border incident. Maximum additional critrus and beef imports would not exceed $500 million - hardly a dent in Japan's current $18 billion trade surplus over the United States ($6 billion more than last year).

To foreign-service professionals in the U.S. embassy, all this smacks of "Jap bashing." However emotionally irrational it may be, the stakes are enormous. There is real fear here by both government and business of a protectionist trade war set off by Congress that would be a tragedy for the West and sheer disaster for Japan, which must trade to survive.

Why not try to keep this tiger caged by dropping the last agricultural import quotas? That is a question asked by influential japanes businessmen, who grumble about Prime Minister Takeo Fukuda's caution. But the malaportioned farm vote is so vital to Fukuda's conservative LDP that he must tread carefully. Nor would his deafeat by factional rival Masayoshi Ohira in the current LDP leadership election make the slightest difference.

In the absence of dramatic action ending agricultural quotas, the Fukuda government hopes and prays that serious efforts to hold down Japanese exports will soon be reflected in the U.S.-Japanese trade balance. But that may be too late to forestall a runaway protectionist Congress.

In the meantime, anger has begun to develop here. "Don't push us too hard," warned an angry business executive, normally pro-American, after hearing of the threats by the three visiting congressmen. Such Japanese leaders feel Americans ignore Japan's painful efforts to curb its export mentality at a time when Japanese shipyards are closing the steel mills are laying off workers.

To them, fomenting a world crisis over a few oranges in madness, considering common strategic interests to the two nations in the Western Pacific amid inexorable growth of Soviet power.