Although aware that political winds in the United States are shifting sharply against them, Japan's automakers appear reluctant to support broad, industrywide restrictions on their exports into the sagging American market.

Their public posture is that time, rising Japanese car prices, and the American industry's new small cars soon will put natural limits on their exports and their growing share of American sales.

Registration of Japanese-made cars sold in the United States declined during three recent months, and the appreciation of the yen will raise their prices there, claims Masataka Okuma, executive vice president of Nissen Motor Co., which produces Datsuns.

"Judging from this situation, Japanese car sales in the United States will become normalized," he said in a telephone interview recently. "Any form of a volume restriction, even a voluntary one, in my opinion will not contribute to easing the employment situation in the United States."

There was prominent speculation in Tokyo last week that new American political pressure, coming in part from President Carter, might force the Japanese industry to restrict exports voluntarily.

But a flurry of denials left the government and automakers in their traditional positions. The Ministry of International Trade and Industry (MITI) agreed to suggest that the companies export in an "orderly" manner, and the companies promised to exercise prudence in the coming months.

Nevertheless, there were distinct signs that some measures might be taken soon to counter growing U.S. discontent with Japan's export surge. "Voluntary restrictions are not acceptable in business circles," said one official, "but there is agreement on the political necessity to do something."

The comments of a Toyota Motor Sales Co. spokesman, Toyoo Okada, implied that his company expects some form of government directive on exports. Because of the large number of American auto workers laid off recently, it is possible that Japan will take some "counteraction," he said.

The companies themselves cannot restrain exports without running afoul of U.S. antitrust law, he said, but if the government acted first Toyota would be forced to go along.

"Our position is that if the government asks us to do something there is no way we could rebel," Okada added.

A story in the authoritative economic journal Nihon Keizai last week indicated that some directive might be forthcoming soon from the MITI. It suggests that the ministry virtually has decided on a system of voluntary restraints imposed under Japan's export-import control law which would require automakers to cut back to a fixed level of exports. A MITI official categorically denied the story, and the ministry subsequently announced it merely would urge each company separately to exercise moderation.

Several recent developments in Washington prompted the reconsideration here. One was Senate passage, by a 90-to-4 vote, of a resolution which criticized Japan for planning to expand exports further. Another was the United Auto Workers petition to the International Trade Commission asking for import restrictions and an announcement that Ford Motor Co. may file a similar petition.

There also has been press speculation that President Carter, under the influence of campaign pressure in auto-manufacturing states, may abandon his current position and press Japan to adopt export restraints.

That suspicion was fueled by reports that Carter, during the Venice-summit, had asked Japanese officaials for an explanation of reports that Japan's automakers were gearing up for a massive new export offesive that would increase shipments to the United States by 2 million vehicles in the next seven years. Japanese officials and industry spokesmen denied any such plans.

Japanese car exports to the United States have been soaring this year as Americans turned to the smaller autos that use less gas. During the first four months, sales of both Japanese cars and trucks were about one-third higher than in the comparable period of 1979.

American critics have claimed the export intrusion would not wear away even when U.S. manufacturers came out with their smaller versions and that Japan would be able to claim more than a fourth of the American market permanently.

The Japanese industry claims that the export advantage is short-lived and will fade this fall. Nissen's Okuma, for example, said that the number of Japanese cars registered in the United States in May was nearly 20 percent lower than in the same month last year.