Nakasone tries to break Japan's political stalemate with election.

Seemingly bowing to the inevitable, Prime Minister Yasuhiro Nakasone has decided to call an early general election to end a period of strife that has stymied Japanese politics.

If the opposition parties have their way, the central campaign issue will be ''political ethics'' and the behavior of Mr. Nakasone's mentor, former Premier Kakuei Tanaka.

In a meeting with the heads of both houses of the Diet (parliament) on Saturday, Nakasone indicated his intention to seek a new mandate for his year-old administration. Political sources say Dec. 18 almost certainly will be the date if the opposition parties agree.

The lower house term actually does not expire until next June. But for weeks rumors had been rife that the electorate would get an early chance to pass judgment on the state of national politics. This was mainly because of Tanaka's conviction Oct. 12 for accepting a 500 million yen bribe while he was premier from the US aircraftmaker Lockheed Corporation to help promote their sales in Japan.

Until Saturday, however, Nakasone, who was virtually alone among top executives of the ruling Liberal Democratic Party (LDP), had made no comment on the election issue. Political analysts orignally felt he would have preferred to wait at least until January or even longer to give the voters more time to forget about Tanaka's conviction.

Although Tanaka resigned from the ruling party in 1976 on his arrest in the Lockheed case, he remains the most powerful politician in the country. He has the allegiance of at least one-third of the LDP Diet membership and seemingly unending reserves of political funds. He was instrumental in pushing Nakasone to power last November, despite widespread mistrust of Nakasone's hawkish policies on defense and his ''weathervane'' ability to shift direction with the prevailing political winds.

After receiving a four-year jail term - which he is now appealing - Tanaka doggedly refused to give up his Diet seat, causing opposition parties to introduce a Diet resolution demanding his resignation. Nakasone refused to accept this, provoking an opposition boycott that has already caused a one-month hiatus in Diet proceedings.

The premier justified his action by saying the Constitution guarantees the status of Dietmen, so nothing could be done to unseat Tanaka if he did not voluntarily resign. Nakasone gently suggested such a move in a recent meeting with Tanaka, who firmly rejected it.

Nakasone, however, has now agreed that ''in view of the unusual situation (in the Diet), there is a need to refresh the people's minds as quickly as possible.''

The opposition is expected to focus on corruption in the ruling party as typified by Tanaka. Nakasone, on the other hand, is thought likely to stress the government's attempts to revive the economy and alleviate the public's financial burden through such measures as a recently announced income tax cut - the first in seven years. He is also expected to try to capitalize on his growing international stature, through meetings with world leaders, such as those just held with President Reagan. (The US President left Japan the day Nakasone made his election decision public. Many people feel the successful four-day visit brought the US and Japan even closer than before.)

The ruling party has 286 seats in the 511-member lower house and some losses are considered fairly inevitable in the next election. It is Nakasone's main task to minimize these to protect his stature within the LDP and hold down interfactional feuding in the party.

Some LDP Dietmen are predicting that up to 20 seats may be lost. Even so, the party that has ruled Japan virtually throughout the postwar era would still retain power.

The splintered opposition parties are weak. They hardly offer an alternative. In the final analysis, no matter what they may think about the ethical questions surrounding Tanaka, many Japanese likely will vote for the LDP to protect their economic livelihood under the principle of ''better the devil you know. . . .''