The U.S. love/hate relationship with the Japanese is reaching a critical juncture.

On the one hand, the trade bill President Reagan has vowed to veto still contains onerous Japan-bashing provisions.And Democratic leaders, who shepherded the measure through Congress, have made it clear they welcome a veto to create a vote-rallying issue in November.

On the other hand, a leading Democratic thinker who might be expected to influence the platform that emerges in Atlanta is talking about even closer cooperation with on our No. 1 Pacific ally.

He is Zbigniew Brzczinski, former President Carter's national security adviser. Writing in the current issue of Foreign Affairs, he urges a new U.S.-Japanese partnership:

It is really quite striking how much the two countries' needs and interests match. The strengths of one compensate for the weaknesses of the other. Each needs the other; indeed, each is likely to falter without the other.

Mr. Brzczinski says the United States needs Japanese capital to finance industrial renovation and technological innovation. It needs Japanese cooperation in protecting its still-significant global lead in research and development. It needs Japanese participation in the economic development of such threatened areas as the Philippines, Pakistan, Egypt, Central America and Mexico.

Japan needs continued U.S. security protection, open access to the U.S. market and, through cooperation with the United States, secure access to a world market. It also needs to maintain and expand its collaborative participation in U.S. corporate and academic research.

Without the well over $100 billion of capital inflow from Japan over the last several years, Mr. Brzczinski adds, the U.S. economic situation might have become untenable.

On the other hand, without U.S. protection Japan probably would have had to spend $50 billion a year on defense, while alienating and frightening its neighbors.

Some of these thoughts are taken a step further by Albert Wojnilower, senior adviser to the First Boston Corp., investment bankers, in an interview in The International Economy magazine.

While the United States grows closer to Japan, he says, it moves farther

from Europe, particularly West Germany. The reasons? U.S. and Japanese agreement on a full employment goal and West Germany's unwillingness to do so. Also, Europe's satisfaction with an internal market and its resulting protectionist stance.

Mr. Wojnilower sees Western Europe growing closer to the Soviet Union and Eastern Europe:

The Soviet bloc has a good opportunity to become for Europe what Latin America used to be for them and for us - an outlet for their production. Such a relationship would lead to mutual dependency, which, so long as the Soviets maintain a smiling face, is apt to be intrinsically appealing to the Europeans.

Greater U.S.-Japanese collaboration is by no means certain. Japan bashing, a modern-day reflection of the Yellow Peril concerns of a century ago, remains alive and virulent.

The Garn amendment to the trade bill is an example. Largely ignored in the United States, the provision is viewed in Japan as an indication of U.S. hostility.

The provision, already watered down, would penalize the giant Toshiba Corp. for a sin of one of its subsidiaries, the sale to the Soviets of milling equipment to help produce silent-running propellers for their submarines.

Sam I. Nakagama, head of an economic consulting firm that bears his name, says the issue has been blown completely out of proportion.

An American of Japanese descent, Mr. Nakagama served in the U.S. Army in World War II and on Gen. Douglas MacArthur's staff during the occupation of Japan.

He notes that Soviet submarines with quiet propellers appeared three years before Toshiba began its shipments; that Italian, West German and French companies had sold similar equipment to the Soviets, some before Toshiba; that quiet propellers play only a small part in helping a submarine run free from detection; and that a milling machine can only rough cut a propeller and by itself cannot make it quiet.

The point is clear: Increased U.S.-Japanese cooperation may be in the interest of both countries, but politicians in this country must be willing to forgo short-range political advantage if it is to get a try.