Japanese premier survives party challenge - but not unscathed

Prime Minister Yasuhiro Nakasone has been assured of reelection to a second, final two-year term as party president (and hence as premier) after nearly losing it because of a party revolt that surfaced at the last minute.

Mr. Nakasone's position within the ruling Liberal Democratic Party (LDP) has been considerably weakened thereby, as has that of his chief supporter, former Premier Kakuei Tanaka. Mr. Tanaka, whose power is such that he is often called the ''shadow shogun,'' is currently appealing a lower court conviction on a charge of accepting a $1.3 million bribe in the so-called Lockheed scandal case.

Recent polls show that Nakasone, with his rugged good looks and spellbinding oration, is supported by well over half the electorate. That is a very high proportion for this country, where people tend to be bored with the LDP, which has been in power for almost all the postwar years.

Nakasone has committed no obvious major mistakes during his two years in office so far, nor has any scandal touched his administration. Even his enemies acknowledge that the prime minister has given Japan a voice in world affairs such as it never enjoyed before, conducting personal diplomacy with his good friend Ronald Reagan and other world leaders.

Under LDP rules, the president is elected every two years and can have only two terms altogether. The Liberal Democrats have an alliance with a small splinter party, the New Liberal Club, which gives them a comfortable majority in the Diet (parliament) and assures their president of the prime ministership.

Nakasone's term expires at the end of November, and the deadline for filing candidacies was to have been Monday. As the weekend approached, most political observers believed either that Nakasone would have no rivals (in which case he would be reconfirmed by acclamation) or that, if it came to an election, he would win by a convincing margin.

But when a council of top party officials convened Saturday, scathing criticism of Nakasone came from an unexpected quarter: Susumu Nikaido, vice-president of the party and chairman of the Tanaka faction, the largest and most powerful of the party's five major factions. Nikaido apparently had a dispute with Tanaka Saturday morning, telling him, ''I will not follow you if you are wrong.''

Other, known opponents of Nakasone, including former premiers Takeo Fukuda and Zenko Suzuki, chimed in.

While various leaders privy to the plot to unseat Nakasone had their own motives, the point on which all agreed was the need to undertake thorough party reform, by which they meant the elimination of Tanaka's influence in party affairs.

Eventually, all Nakasone's declared rivals agreed to step down if that were the party executive's wish.

Nakasone's immediate task is to reconstruct his Cabinet and to form a party executive that will be distributed fairly among the five factions. The Tanaka faction is still likely to get the powerful post of secretary-general, which controls the party purse strings. But its influence is bound to decline. Nikaido's challenge to Tanaka has brought to the surface a fissure that will not easily be closed.