A theory popular among foreign observers holds that the Japanese voter likes to have opposition parties around so long as they are not allowed to come near the real levers of power.

The theory got a new lease on life today. The Japanese voter showed that he really is delighted with the Liberal Democratic Party, warts and all, and that the usefulness of opposition parties ends when they seem to be on the verge of sharing control of the government.

Throughout much of the year, the polls and pundits had asserted that the "age of coalition" was upon Japan and that the crusty, conservative Liberal Democratic Party would have to yield some of its power. It was too corrupt and its leadership was too old and too much given to bickering to merit the popular trust, the experts decided.

The predictions went down the drain today when the votes were counted and the Liberal Democratic Party emerged with its biggest lower-house majority in years.It can proceed to pick the next prime minister and run the government without any interference from opposition parties, some of which were severely trounced.

The election was supposed to be a referendum on the Liberal Democratic Party and its chain of proven corruption that stretched back to the Lockheed scandal and former prime minister Kakuel Tanaka. So what happened? Tanaka was reelected handily and will play a big part in selecting the next prime minister. The former justice minister who had him arrested in 1976 was defeated.

The final tallies showed that the Liberal Democrats won 284 lower house seats, 36 more than they won last fall and 28 more than the simple majority needed to control the chamber and elect a new prime minister. The party's margin will grow even bigger when several candidates elected as independents switch their allegiance to the Liberal Democrats.

The principal opposition, the Japan Socialist Party, barely managed to cling to the 107 seats it held when parliament was dissolved last month. For other parties, the day was a disaster. The central Komeito, the so-called "clean government party," suffered a stunning loss of 25 seats and the Communist Party lost 12.

The Liberal Democratic Party also easily won control of the less powerful upper house, but the final votes in that contest will not be counted until Tuesday morning.

A number of theories were used to explain the unexpected margin of victory. Some attributed it to a sympathy vote for the late prime minister Masayoshi Ohira, who died of a heart attack 10 days before the polls opened. Ohira's death, Democratic Socialist Party Chairman Ryosaku Sasaki said, aroused the emotion of voters and served to obscure the corruption issue.

Socialist Party Chairman Ichio Asukata, who barely hung on to his own seat, agreed. The premier's death helped the Liberal Democratic Party tighten its own ranks and fogged the policy debates, which his party had hopes would carry it to power.

The whole concept of coalition politics in Japan was given a bad name by the overwhelming defeat of those who had angled for some kind of power-sharing arrangement. The election campagin disclosed that the parties of the left were not very close together on such key issues as national defense and it was hard to see how they could govern in a coalition arrangement.

Whatever the reason, the results delighed big business, which has more at stake in the Liberal Democratic Party's success than any other segment of society. Early in the afternoon, when the outlines of the victory became clear, the stock market went up and the yen rose sharply against the U.S. dollar. Yoshihiro Inayama, head of the major association of large corporations, proclaimed that he was happy and satisfied that Japan "now has stable politics." Eishiro Saito, president of the Iron and Steel Federation, said the results reflected support for Japan's free-economy system.

Safe and sound with what the late prime minister Ohira always wanted, a "stable majority," the Liberal Democratic Party can now return to its major task, choosing a new prime minister. Party leaders meet Tuesday to begin working on procedures for that choice and a new premier, at least a temporary one, will probably be selected in mid-July.

For nearly a decade, that choice has been a product of nasty in-fighting among four or five major Liberal Democratic factions, each of which wants the power, prestige, and money that comes from having one of its own at the top of the government.

Much was heard this year about dissolving the factions, whose unseemly feuding clearly irritated the public. Most observers doubt that the dissolution movement will get very far and believe that the man who can put together the most factional support will win, as usual.

The results today show that the factions led by Tanaka, the associates of Ohira, and by Yasuhiro Nakasone, a candidate for premier, will have the greatest power if they can stick together. Tanaka emerged again as kingmaker, or close to it.Although expelled from the party in the mid-1970s, he still manipulates the party's largest faction.

If Tanaka throws his support to Nakasone and is able to bring along his and the Ohira faction, Nakasone is likely to emerge as the favorite. But some in Tanaka's own faction are threatening to bolt if he tries to do that.

An informal tally shows the Tanaka-Ohira-Nakasone factions with 151 votes compared to about 95 for factions led by former prime ministers Takeo Fukuda and Takeo Miki and by Ichiro Nakagawa. The second cluster of factions is expected to support Toshio Komoto against Nakasone.