Washington — At home and abroad, President Reagan this week is walking a fine political line between moderation and his more conservative instincts on national defense. For the moment, he is more observer than active participant as the United States Congress and international forums - European allies, the United Nations, and the International Civil Aviation Conference - decide how to respond to the South Korean airliner tragedy.
But administration firmness on military matters - particularly as they relate to Moscow and its allies - is evident nonetheless. Three more US ships and 2,000 US marines arrived off the coast of Lebanon as Syrian-backed Lebanese Muslim forces increased military pressure on the Lebanese government. White House spokesmen said that there were no plans to reconsider the administration position on whether the War Powers Act applies and that Congress should have a say in the growing US military presence there.
In a speech to the Air Force Association convention in Washington Monday afternoon, Defense Secretary Caspar W. Weinberger reemphasized administration resolve on national defense and foreign policy matters as well.
Just back from visiting American advisers and US-trained troops in Central America, Mr. Weinberger continues to press for Reagan administration programs - particularly a continued increase in defense spending and such controversial weapons as the MX intercontinental ballistic missile and chemical weapons.
With fresh evidence of expanding Soviet military might and the willingness to use it, Weinberger asks critics of increased defense spending where they would cut, and what foreign policy commitments they would renounce in order to save money.
Another senior Pentagon official, Undersecretary for Policy Fred Ikle (No. 3 in the Defense Department hierarchy), also used a public forum Monday to vigorously defend the administration's policy in Central America.
Mr. Ikle told the Baltimore Council on Foreign Relations that without more support for Nicaraguan rebels and the governments of El Salvador and Honduras, US security would be harmed and an increasingly dangerous East-West military confrontation result. He was particularly critical of lawmakers who have curbed such support.
Returning lawmakers are quick to back the President's measured response on the loss of the South Korean airliner. But in other areas related to national security, there will continue to be more controversy.
The $187.5 billion defense authorization bill worked out by House and Senate conferees will not be approved without a fight. In particular, opponents are incensed that the bill contains $155 million for production of new chemical weapons, an item previously cut by the House and kept alive in the Senate only by a vice-presidential tie-breaking vote.
Former President Nixon halted the production of nerve gas in 1969, and no new chemical bombs or shells have been built since then. Citing the suspected use of ''yellow rain'' by the Soviet Union in Afghanistan and Southeast Asia, the Reagan administration wants to spend $6 billion over five years rebuilding the US chemical arsenal.
Defense appropriations panels - those committees that actually allot money for military spending - begin their work this week. While the South Korean airliner episode is expected to strengthen the President's hand, the Pentagon will continue to have an uphill route to increased budgets of the type sought by the White House.
The President finds himself in the unusual position of being criticized by Democratic presidential hopeful Walter Mondale for not being tough enough with Moscow, and being defended by Sen. Barry Goldwater (R) of Arizona - ''Mr. Conservative'' - from attacks of ''paralysis, confusion, impotence, and fear'' by the New Right that helped elect him.
In a Time magazine interview this week, Reagan speaks with frustration of the ''great limit'' on his options in response to the Soviet downing of the South Korean airliner.
Underlying it all - particularly with a likely reelection bid next year - is the unspoken realization (as military leaders and the most respected students of Vietnam these days are saying) that any sharp stiffening of defense posture or escalation in military activity abroad must be accompanied by clear public understanding and support.