One of President Reagan's most important stops on his Asian trip next month will also be one of his most challenging - considered in terms of US domestic politics. That will be the President's visit to Japan. The two industrial superpowers have worked out a close and mutually beneficial international and economic partnership.The immediate task for both nations is to ensure that nothing interferes with that degree of common purpose that has been achieved.
In this regard, speculation mounts that the bribery conviction of former Japanese Prime Minister Kakuei Tanaka could inhibit an early resolution of major trade-related disputes between the two sides. Why could that occur? Because, analysts suggest, current Prime Minister Nakasone, who was close to Mr. Tanaka, will now need to prove his independence to the Japanese electorate. But doing so could preclude steps that might be perceived as ''giving in'' to the US on trade discussions.
At the same time, President Reagan, faced with calls for tougher action toward Japan from US labor unions and several Democratic presidential contenders , is under pressure to prod Japan into restricting its auto exports while accepting more US imports.
Both nations have everything to gain from crafting the best possible accommodation. Unfortunately, sharp differences remain. Among them: whether Japan should accept more US farm exports (beef and citrus); whether Japan should limit auto exports; the extent to which Japanese firms deliberately engage in industrial theft regarding trade secrets of some US firms, particularly computer companies; how to reduce Japan's massive trade surplus vis-a-vis the US (expected to be in excess of $20 billion this year).
The three-year ''voluntary'' auto pact (reached after sharp pressure from the US, under which Japan is limited to 1.68 million car exports) expires next March.
For Mr. Nakasone to publicly embrace an extension could be politically difficult. Thus, it would seem prudent for the US to be the first to formally call for an extension - if an extension is felt absolutely neccessary to ensure the survival of the US auto industry. Still, given the recovery, the US should certainly be able to raise the ceiling and allow a higher number of exports, say 2 million or so cars. General Motors, which wants to purchase subcompact cars from Japan, favors hiking the quota.
In a sense, the very friction that now and then arises over US-Japan trade underscores the degree to which the two nations have become interdependent.
Mr. Reagan's task will be to convey that sense of mutual interdependence when he visits Tokyo.