Japan's ruling party struggles to escape the shadow of a 'shogun'

Japanese politics has reached a crucial, possibly dangerous postwar turning point with the conviction of former Prime Minister Kakuei Tanaka for accepting a bribe.

In a verdict widely expected after a seven-year trial, Tokyo District Court Wednesday imposed a four-year jail sentence and a 500 million yen ($2 million) fine on a man who has remained immensely powerful throughout a decade of scandal.

In an unprecedented legal decision, Mr. Tanaka was convicted of receiving while prime minister a 500 million yen bribe from the Lockheed Corporation to use his personal influence to persuade a domestic airline to buy Lockheed passenger aircraft.

The three-man panel of judges said the case had ''seriously hurt the people's confidence in politics, betrayed public trust in fair public service, and caused an immeasurable impact on society.''

One newspaper put it in a wider context of a judgment on ''money politics, deeply rooted in this nation's political world and most eloquently symbolized by Tanaka.''

Another said: ''(The ruling) is bound to have great repercussions, shedding light on the shady aspects of Japanese politics long dominated by Tanaka-style politics relying heavily on the power of money.''

But in many ways the judgment has merely added to the confusion.

Tanaka, and three of four other defendants who received jail terms for channeling the bribe to him, immediately filed appeals. Legal experts say it could be 10 years before the appeal process is completed. And some observers wonder why Tanaka was not given the maximum sentence of five years' imprisonment.

But the pressing questions concern the course of Japanese politics:

* Will Tanaka finally accept political responsibility and resign from the Diet (parliament)? His behavior over the past nine years does not suggest he will. In a statement read after the court's sentencing, Tanaka said, ''It is my responsibility and duty to carry out my work as a parliamentarian.''

* How will this affect the government of Prime Minister Yasuhiro Nakasone, who assumed office last year only because of the immense behind-the-scenes power wielded by Mr. Tanaka? This backroom maneuvering has earned Tanaka such nicknames as ''shogun of the dark'' and ''shadow general.''

There is no easy way out for Mr. Nakasone. His immediate statements and actions seem to suggest he is trying to distance himself from his mentor. Opposition parties and various civic groups immediately launched a determined campaign to use the Tanaka case to promote ''political ethics'' and ''clean out the dirt from the political world.'' If the opposition parties unite in their demand for Tanaka's resignation from the Diet, Nakasone's ruling party will be faced with a political crisis of enormous proportions.

Typical public reaction during the years of Tanaka's trial was that he was merely the man who got caught, and that he was far from being the only politician receiving bribes or engaged in dubious financial practices.

This issue of morality will now dominate Diet proceedings. Many political observers believe this could lead to chaos and force Nakasone to call an early election in the hope of weathering the storm. The prime minister could hope either that Tanaka will quit, perhaps citing ''ill health'' to save face as he did in 1974, or that he will be rejected by his home electorate - a most unlikely occurrence. The evidence so far is he is still regarded there as a ''hero.''

Both Nakasone and Tanaka, however, have many powerful enemies within their own party. This could lead to confusion as various factions fight one another - which would certainly occur if Nakasone did badly in any parliamentary election he might decide to hold.