As the leader of a Communist Party pledged to uphold parliamentary democracy, Kenji Miyamoto has decided that at 68, it is time he entered the Japanese Parliament. The chunky organizational genius who guided the Japan Communist Party through a swift postwar rise is expected ot clinch an upper house seat in Sunday's election.

It is very doubtful, however, that even the Miyamoto magic will be enough to lift his party out of the doldrums. The Communists took a thrashing in December's general elections, it representation in the lower house tumbling from 39 seats to 19.

Campaigning outside a Tokyo supermarket this week. Miyamoto personified what must be the most bourgeois and conservative Communist Party in the world. In a short speech he demonstrated nationalism - attacking the Soviet Union over its possession of four islands claimed by Japan - a belief in ballot-box revolution, and the determination to work within Japan's highly successful capitalistic system. Yet, for many Japanese voters, it simply is not enough.

"I respect Miyamoto himself," said Atsuka Chiba, a housewife, but the Communist are too radical for me." That kind of distrust, a still potent anticommunism in Japan and a fear that with 10 percent of the voting population supporting them the Communists are becoming too powerful have served to stop the Communists dead in their tracks.

Since the early 1950s when a disastrous call to armed struggle cost it all its parlimentary seats, the party has grown spectacularly under Miyamoto's leader ship, first as secretary general, then as chairman.

They doubled their seats at every election and built the best-disciplined force in Japanese politics. The party has nearly 400,000 members.

An economics graduate of Tokyo University, Miyamoto gathered around him a cadre of dedicated, high-caliber intellectuals. In the high-rolling days when Japan's economic miracle was creating minority concerns over pollution, social welfare and political corruption, many ordinary people saw the Communists as reliable champions of their cause.

The party gained a reputation as energetic, well-briefed and ready to challenge the complacent conservative majority in Parliament with pragmatic and frequently innovative policies.

Recognizing the impossibility of promotion gold-style communism in Japan. Miyamoto chose the softer, moderate route of Eurocommunism. He improved the party's image by junking hardcore ideologues of all persuasions. Out went ultra-leftists, pro-Peking and pro-Moscow elements and revisionists. The party dramatized its quarreling bitterly with Communist brethren in China and the Soviet Union. Next to go were all the harsher rhetorical trappings of communism.

The party expanded nationally and locally. Party officials claim 48 million Japanese now live under Communist regional administraions.

The counter-attack came last year when the conservatives and all other Japanese political parties ran a "red menace" campaign aimed at the Communists. The ensuring rout at the polls showed the public does not believe the party's profession of faith in human freedoms and the democratic process.

Some analysts assert that the Communists have peaked politically. Political Science Prof. Rei Shiratori insists there is no chance of the Communists gaining power in Japan for at least 30 to 40 years.

"In the past people tried to balance the strong conservative government by voting Communist," he said. "But the conservatives were very successful in selling the idea that a sound economy defends on them staying in power."

Ironically, now that there is real fear that low or stable growth will cause poverty or recession. Japanese voters are less inclined than ever to trust their fate to the Communists.

Constantly trimming its policies to appeal to an almost universally conservative their fate to the Communists.

Constantly trimming its policies to appeal to an almost universally conservative electorate, the Communist Party has now drifted across the political spectrum to somewhere on the British Labor Party's left.

"We don't want to rapidly change people's lifestyle," said Vice Chairman Koichiro Ueda. "People don't want political change, they want economic changes."

The party is also moving away from its former antagonism towards the United States. While they still call for abrogation of the Japanese-U.S. Mutual Security Treaty and a neutral Japan, the COmmunists emphasize that they will not embrace China or the Soviet Union.