West Germany has embarked upon an intense public debate on its defense and security policies amid increasing restiveness within Chancellor Helmut Schmidt's own political party and uncertainty here over the strident tone of the Reagan administration toward the Soviet Union.

Such a discussion of defense issues is rare in this key member of the NATO military alliance, where for years the public has preferred not to talk too directly or extensively about security affairs, particularly about the communist threat from the east.

In the face of a heightened attack from the left wing of the ruling Social Democratic Party, however, aides to Schmidt are going to unusual lengths to prepare a public campaign aimed at defending Bonn's commitments to NATO and to a strong defense as a deterrent to the Soviets, according to sources in the chancellor's office.

While a political necessity now for Schmidt, such public discussion at this time also underscores the differences in national mood and official policy between West Germany and the United States, raising the potential of fresh transatlantic controversy and misunderstanding.

Already, harsh statements directed at the Kremlin by President Reagan and Secretary of State Alexander Haig Jr., together with remarks by Defense Secretary Caspar Weinberger reopening discussion of the neutron warhead issue, have produced uneasiness here and drawn some critical comment. Some critics read the administration's pronouncements as suggesting a lack of U.S. interest in proceeding quickly with arms control talks.

Leading Social Democratic legislators have warned publicly in the past month that reluctance on the part of the United States to pursue arms control might force West Germany to reconsider its support of the 1979 NATO decision to station U.S.-made, medium-range nuclear missiles in Western Europe. That decision was linked at the time to an invitation to the Warsaw Pact to open talks with NATO on limiting such weapons.

Much of the reason for Bonn's highly sensitive reaction to what other major West European governments have taken largely as initial public posturing by the new U.S. administration is Schmidt's challenging domestic political situation.

Schmidt is at odds with large factions of his party on a range of important issues, from defense and economic policies to nuclear energy and workers' rights. Disagreements among the Social Democrats that have smoldered for years have burst into the open lately, despite traditional party discipline and compromise.

One of the party's deputy chairmen, Hans-Jurgen Wischnewski, recently described the internal squabbling as the worst in the party's postwar history. Herbert Wehner, the party's parliamentary leader, warned that the quarrels could rupture the party.

The most important split, affecting West Germany's credibility as a partner in the Atlantic Alliance, is over the NATO decision. Additionally, several dozen left-wing Social Democrats in parliament favor trimming the defense budget to bolster funds for development aid, and an even larger group opposes the proposal sale of West German tanks to Saudi Arabia.

While the left-wing dissidence is rooted largely in antimilitaristic party ideology, the chancellor's aides say, too, that some of the critics simply do not understand the facts and assumptions of defense strategy. This, on top of a recent outpouring of mail to the Foreign Ministry indicating confusion and doubts about Bonn's foreign policy, has moved the government to plan a more forceful and detailed explanation of its position.

"We are perhaps paying now for not having had a broad discussion on defense and security policy for a number of years," a Schmidt aide said. "No one has really explained what our strategies are."

The current debate on defense issues comes amid a discernible growth of the undercurrent of pacifist sentiment in West Germany. Bonn government officials -- among them Defense Minister Hans Apel -- worry about a loss of confidence in the strategy of deterrence.

In the many, often heated discussions on security policy in West Germany today, aides to Schmidt observe a public sense that no workable military protection exists and that new NATO weapons simply will attract more Soviet weapons. The very numbers of nuclear warheads on West German territory are said to be enough to cause widespread public fright.

But what concerns many informed observers goes deeper. They worry that the West's concept of deterrence has been made questionable by the development of sophisticated technology that has also strengthened the capability to wage war. They insist on a public airing of defense policy unlike anything seen here since the 1950s, when the decision to rearm was taken.

"There was no strategic debate here in the 1960s and 1970s, if you leave Schmidt himself out of it," said Karsten Voigt, spokesman for the Social Democrats' parliamentary group on foreign policy and a moderate voice on the party's left. "This has changed now. We are participating in the debate."

The fanning of open discussion is sure to leave West German politics unsettled for some time, with expected spillover effects into smaller European governments facing similar challenges, such as NATO partners Belgium and The Netherlands.

Furthermore, the Social Democrats' political difficulties pose a threat to their coalition with the small Fee Democratic Party, headed by Foreign Minister Hans-Dietrich Genscher. Some Free Democrats -- particularly outside Bonn -- speak of a realignment with the more conservative Christian Democrats if the Social Democrats fail to get their house in order.

The Free Democrats, thanks largely to an impressive performance in last October's general elections that was mostly responsible for the coalition's gain of 35 seats in parliament, have been exercising correspondingly greater influence on coalition policy. Left-wing Social Democrats worry that the strong pro-Washington, probusiness leanings of Genscher's Free Democrats will tilt the government too far from Social Democratic principles.

Many of the Social Democrats' problems are attributed to a lack of party leadership by the party chairman, Willy Brandt, and by Schmidt.

In fact, the party's troubles have tied the hands of two men who had been touted as Schmidt's successor -- former justice minister Hans-Jochen Vogel, who was dispatched to be mayor of West Berlin when a political scandal there rocked the previous leadership, and Defense Minister Apel, who is embroiled in controversy over a major cost overrun for the Tornado fighter plane.

"The Bonn coalition won't come apart," observed an American diplomat. "But what we don't know is whether it will limp along or not."