TOKYO (AP) _ A heavy loss in a parliamentary by-election has darkened the clouds gathering over Prime Minister Kiichi Miyazawa's administration, raising questions about the future of his long-governing conservative party.

If Miyazawa is forced to resign by summer, as some observers predict, he would become the third of the last four prime ministers since 1989 to step down amid scandal.

''During this time the United States, for example, had only one president, George Bush. How weak and shameful Japanese politics is 3/8'' exclaimed one commentator, Kenzo Uchida, in a Japan Times column.

Perhaps the deepest fear in governing circles is that the almost feudal character of Japanese politics - in which power is fashioned through patronage jobs and cash gifts, giving rise to chronic scandals - is hindering Japan from taking a world leadership role at a time it should be.

The latest in a spell of money-for-favors scandals that have blackened the governing Liberal Democratic Party's image was the focus of a campaign that ended Sunday with an overwhelming defeat for the party's candidate.

The election, for an Upper House seat representing Nara in western Japan, was also considered the first national evaluation of Miyazawa's three-month- old administration.

The Liberal Democrat candidate, Nobuharu Enoki, tallied just 178,002 votes to 244,930 for the winner, Yukihisa Yoshida, who was backed by the powerful Rengo labor federation and Socialist and Democratic Socialist parties.

''My victory means the people will no longer tolerate corrupt politics,'' said a jubilant Yoshida, who ran on an anti-corruption platform.

The Mainichi newspaper seemed to agree, editorializing that the election results indicate ''voters decided that the Liberal Democratic Party equaled bribery.''

Miyazawa, whose administration has been tarnished by scandal testimony in parliament and diplomatic gaffes vis-a-vis the United States, called the results ''truly regrettable.''

''I humbly accept the results of this election and ... will continue my efforts to win back confidence in politics and improve living standards for the people through promotion of political reform and facilitation of budget deliberations,'' he said.

But whether he can muster the political strength to reform the political system is deemed doubtful. And if opposition parties continue to stymie current debate on the budget for the fiscal year that starts April 1, Miyazawa may have to be sacrificed as was one of his predecessors, Noboru Takeshita.

''LDP Fears Repeat of Nightmare,'' read one headline today in the leading Asahi newspaper.

It referred to Takeshita's forced resignation in 1989 when the budget process was disrupted by the notorious Recruit scandal. His successor, Sousuke Uno, then resigned to take responsibility for an Upper House election defeat.

Miyazawa faces a similar scenario.

If the governing party loses two more by-elections this spring and is defeated in a general Upper House election in July, it could easily serve as ''an ignition point for a change of rule or a general restructuring of the political system,'' another paper, the Tokyo Shimbun, predicted.

The prime minister, however, may yet survive. Unlike Takeshita, he has not yet been personally implicated.

In addition, given the weakness and disorganization of the mostly leftist opposition parties, the powerful Liberal Democrats could make a comeback at the polls as they did following Uno's resignation.

The party has governed Japan for 37 years.

Even so, the outcome of Sunday's election was seen as an endorsement of opposition demands that leading members of the Liberal Democrats testify in parliament on ongoing bribery scandals.

Opposition parties began boycotting budget deliberations in parliament last Wednesday after the Liberal Democrats rejected demands that a former prime minister and 17 others testify in a new scandal involving a former close ally of Miyazawa's.

Miyazawa, a 72-year-old former statesman, now finds himself bracketed by such scandals. Opposition lawmakers also want to revive testimony on the notorious Recruit scandal that in 1988 forced Miyazawa, then finance minister, to resign.

And another, potentially larger scandal involving a parcel delivery company that allegedly paid lavish sums to senior politicians is under investigation.

Kiyoshi Yoshikawa, an aide to Enoki, the defeated Liberal Democratic candidate, conceded that ''the scandals served to our campaign's disadvantage.''

''We fought this campaign knowing full well that more than just one seat was at stake as far as the future for the Miyazawa administration,'' he said.