Yasuhiro Nakasone is a politician fortunate enough to see the issues turn his way just when he needs them.

Long a spokesman for bigger defense spending, Nakasone used to be regarded a bit warily by others in the Liberal Democratic Party. A higher military budget was something that polite politicians did not talk much about in a nation where pacifism is inscribed in the constitution.

Now, Nakasone, an all-but-announced candidate to replace the late Masayoshi Ohira as prime minister, senses that the Japanese are more willing to hear his thoughts on defense, and he is not bashful about telling them what he thinks.

He tells audiences it is a dangerous folly for Japan to remain unable to protect itself. If threatened today by a foreign military power, Japan would be in "chaos," he warns.

He recalls derisively the recent comment of a Japanese academician who said there would be no choice but to wave a white flag of surrender if Japan were invaded. "I cannot believe he is Japanese," says Nakasone. "We will defend Hokkaido, Kyushu, Skikoku and Honshu by all means. We are not cowards."

In an interview tonight, Nakasone said that the Japanese have been affected by a change in international conditions.

"The Japanese people are now aware that the power of the Soviet Union has been increased and that the balance of power between the Soviet Union and the United States has changed," he said.

A growing Soviet naval force in the Pacific and more Soviet troops stationed on Japan's disputed northern islands are couple of reasons for the change, he said. So was the Soviet invasion of Afghanistan, which was a sobering jolt for many Japanese who had gone along with the postwar aversion to armaments.

Nakasone is campaigning now for friendly conservatives seeking seats in the parliamentary elections this weekend. Custom forbids him from mentioning it, but he is also a candidate to succeed Ohira, who died Thursday. ("It is like climbing Mount Everest," is the closest he will come to discussing the premiership. Does that mean he is a candidate? "All mountaineers want to climb Mount Everest" is the reply.)

The trim, energetic Nakasone, 61, is one of three Liberal Democratic Party members who are usually mentioned as likely candidates to head the government. Their fight will begin in earnest once the parliamentary election is over. For the moment, they all keep their gloves on -- in public, at least. Yet his long advocacy of military strength will be a help for Nakasone when the balloting for prime minister begins, possibly next week.

Nakasone speaks of modernizing and improving the quality of Japan's defenses and warns of a shortage of new military research and technology. But he dodges questions that ask specifically how much he would increase defense spending.

The defense debate usually revolves around whether Japan should increase its military budget from 0.9 percent to a full 1 percent of the gross national product. Nakasone refuses to pick any figure. He once talked of a "comprehensive" security budget of 3 percent of GNP, but that included such items as food stockpiles, aid to undeveloped countries, and other nonmilitary measures.

Any sizable increase in defense spending would require a tax increase of the sort that was vastly unpopular when proposed last fall by the late prime minister Ohira. Nakasone does not explain how he would finance his defense budget. He told reporters last week that he would be willing to argue publicly for a greater tax burden to finance defense modernization but did not explain how great the burden would be.

Almost alone among Japanese politicians, who favor an oratorical blandness, Nakasone is a melodramatic public speaker inclined toward overstatement. His party colleagues plead gently for votes to preserve stability. Nakasone makes it sound as though the heavens would fall if opposition parties gained a share of power. The stock market would decline, the Japanese yen would fall and Japan would lose credibility abroad if the opposition ran the government, he told one audience.

He promises that the scandal-ridden Liberal Democrats this time will end corruption once and for all and cease factional bickering that has damaged the party's leadership for a decade. One of his favorite targets is the bureaucracy, which he protrays as representing elitist views, not those of the people. He once dramatized the issue by recalling how some rural citizens sold their land cheap to make way for a new highway, but when the ribbons were cut to open the road, high-level bureaucrats dominated the ceremony to claim credit for progress.

Nakasone's main drawback is a lack of popularity among his own party members. He has a reputation for opportunism based on a record of switching loyalties at key moments to advance his own career. Ultimately, his Liberal Democrat colleagues will make the choice of the new prime minister and many of them regard him as too volatile to merit their votes.