The voters' stunning blow to Japanese Prime Minister Yasuhiro Nakasone will restrict his ability to fulfill a promise made to President Reagan to strengthen Japan's defenses.
The Dec. 18 election results did not give Mr. Nakasone the stable parliamentary majority that he wished. He will need to fashion compromises with opposition parties.
With political morality the chief issue of the campaign, the ruling Liberal-Democrats won only 250 of the 511 seats in the House of Representatives and will enjoy a slim majority of 259 after hastily adding nine conservative independents.
A subdued Mr. Nakasone told a press conference Dec. 19 that he hoped to continue as prime minister, emphasizing cooperation with opposition parties and conducting himself with humility. ''The voters' verdict has been very severe. The situation has entirely changed,'' he said.
Mr. Nakasone said he would try carrying out his campaign promises regarding foreign policy, administrative reforms, tax cuts, education. The most important foreign policy pledge was to continue to heighten cooperation with the United States in terms both of defense and of economic relations.
There will be no sudden or abrupt change in Japanese foreign policy. On defense, the national consensus seems to be that Japan needs to be able to defend itself but not to invest the large additional amounts for which Washington has been pushing.
In retrospect, it is clear that the ruling Liberal-Democrats as a whole, and Mr. Nakasone in particular, fought this election campaign with the albatross of Kakuei Tanaka's bribery conviction around their necks. As hard as Mr. Nakasone tried to divert public attention to questions such as tax cuts or education, he could not face down the relentless pressure of the opposition parties and the media on the political morality issue.
Ironically, former premier Tanaka won reelection in his home constituency of Niigata by an unprecedented 220,000 votes. The Tanaka faction also did well nationally, losing far fewer seats than other factions, including that of Mr. Nakasone. This has caused much resentment among the other factions.
Intense backroom bargaining is now going on within the Liberal-Democrat party as well as within the various opposition parties, and between Liberal-Democrats and the opposition. Mr. Nakasone hopes a special Diet session can be convened before the end of the month to elect a new prime minister.
Despite his stunning defeat, Mr. Nakasone is in no immediate danger of losing his job. The Tanaka faction has no candidate of its own. The other factions will grumble and make all kinds of demands. But none of them is ready as yet to try to unseat Mr. Nakasone. Ronald Reagan's good friend ''Yasu'' is therefore likely to continue at the helm at least until next fall when his two-year term as party president comes to an end.