In a listless campaign devold of sharply-defined issues, Japan is preparing to elect a prime minister and it looks as though the 73-year-old incumbent, Takeo Fukuda, will be reelected.

Most observers expect Fukuda to win easily in the novel set of party primaries to be held Nov. 27 and go on to edge out his major opponent, Massayoshi Ohira, in the Liberal Democratic Party's decisive vote Dec. 1. The latter contest could be close.

A string of diplomatic successes, culminating in the widely popular treaty with China, seems to have salvaged the day for Fukuda. A year ago, his Cabinet's popularity was falling and his reelection seemed doubtful.

This is the first trial for a new electoral system in Japan, one that was designed to broaden popular support for both the Liberal Democratic Party and for the prime minister.

The selection remains, however, largely a back-room power struggle among Liberal Democratic Party factions in which personal loyalties and pragmatic political considerations are the decisive elements.

Both Fukuda and Ohira, 68, are bureaucrats turned politicians who began their careers in the finance ministry and gradually built up factional support among the party's members of Parliament.

"Their backgrounds are very similar and so politically they are alike," acknowledges Fukuda's campaign manager, Seijuro Shiokawa. "Perhaps they differ in method, but those differences are minor. So the members are inclined toward Fukuda because he has been in for two years already. They do not like a blank page."

A third candidate, Yasuhiro Nakasone, is the only one striking a clear position on substantive issues. A conservative, Nakasone favors higher defense spending and speaks his mind on issues long considered taboo, including revision of the constitution that was imposed on Japan by U.S. occupation forces. Although he is not expected to win this time, a convincing vote for him would make Nakasone, 60, a prime contender in future years.

Minister of International Trade and Industry Toshio Komoto, 67, is running a poor fourth according to every pool available. The election is actually to choose a Liberal Democratic Party president who will automatically be named prime minister because the party controls both houses of the Japanese Parliament.

In the past, the office has been filled by a vote of only the party's members of Parliament. After the "money-politics" scandals of the early 1970s, the rules were changed to broaden the election process and rid the party of its image of back-room shenanigans.

His year, primaries will be held first in all 47 prefectures, giving 1.5 million party members a chance to state their preference. The two candidates who win the most votes in the Nov. 27 contests will then wage another campaign among the 379 party members in both houses, whose choice on Dec. 1 will be final.

A series of newspaper polls indicates that Fukuda should emerge from the primaries a lopsided winner. A poll by the Yomiuri newspapers found that among the 1.5 million eligible voters Fukuda is preferred by 38.7 percent, Ohira by 23.1 percent.Nakasone by 14.7 percent and Komoto by 5.5 percent.

The vote among party legislators, however, could be much closer, and some observers believe Ohira might still squeeze out a victory. He will get most of the factional voters loyal to former prime minister Kakuei Tanaka. Most of Nakasone's faction votes will go to Fukuda.

The result could be a virtual tie for Fukuda and Ohira. About 75 to 100 uncommitted members will make the difference, and they are being subjected to intense appeals from both camps.

In the past few days, Fukuda's supporters have started a campaign to have the winner of the primaries automatically certified as the next party leader, making the vote among Parliament members merely a formal vote of confidence. Ohira's friends, knowing their only chance lies in factional alliances within the Parliament, reject the idea.

A year ago, Fukuda was considered easy pickings by the opposition gathered around Ohira. With the economy in the doldrums and the high values of the yen frightening businessmen, he looked vulnerable because he was failing to do what he supposedly does best, economic management.

Fukuda has turned the situation around by appearing successfully on the international scene, an arena in which he was presumed to have little competence. Japanese thought he represented the country vigorously at the Bonn economic summit and applauded the tricky negotiations with South Korea to open up oil fields jointly claimed on the continental shelf. The final coup was Fukuda's ability to get credit for the Japan-China peace and friendship treaty, which is being viewed here as the most significant diplomatic event of the postwar years.

Ohira has adopted as a major campaign theme the charge that Fukuda is inclined to use too much "power" in his political dealings, once suggesting that the incumbent displays "authoritarian" leanings. Ohira says he prefers a "flexible" stance, one which would enable him to obtain the cooperation of opposition parties, Fukuda's tactic, he insists, is to "threaten" the oppostition.

Only one substantive issue of any importance seems to divide the two leading candidates. Fukuda has indicated he favors new legislation that would empower defense forces to act in national emergencies if the country is threatened with invasion by foreign troops.

Ohira, on the other hand, contends that there is sufficient law on the books already to permit defense forces act and suggests that what Japan needs is better diplomacy, not more military power.