Shortly before the 1980 election, Ronald Reagan announced in a paid television address that "as president, I will make immediate preparations for negotiations on a SALT III treaty."

After 15 months in office, as was evident at his press conference last night, Reagan continues to favor sharp reductions in U.S. and Soviet strategic nuclear arms, but the "immediate preparations for negotiations" still have not been completed.

The president said last night that he hopes to be ready, "possibly this summer," to engage the Soviets on strategic arms, adding that "it takes a lot of work to prepare for one of these."

Lesser officials said the bureaucracy would need three or four more weeks to complete recommendations for a U.S. negotiating position. Only then will the proposals be ready for consideration by Reagan and other policy makers of the National Security Council.

Over the many months of delay, public alarm about the nuclear weapons buildup and nuclear war has been growing in Western Europe and recently has spread with surprising intensity to the United States. It was to address these fears and the rising pressures for a freeze or early negotiations, officials said, that Reagan in his opening statement to the press conference declared, "My goal is to reduce nuclear weapons dramatically, assuring peace and security."

Why the preparations for strategic arms negotiations have taken so long is a complicated matter, in the view of various officials involved. There seems little doubt, however, that the major factors include the following:

The commitment of many at the top of the administration to a large buildup of U.S. military power, including strategic nuclear strength, and their view that negotiations should only follow such a buildup. This remains the view of many, despite public statements of interest in negotiated reductions.

An initial administration consensus that its strategic arms positions should be dramatically different and more ambitious than those the Carter administration negotiated in SALT II, which Reagan, his political allies and many of those now high in his administration opposed.

The SALT II treaty built on studies and efforts of the Nixon, Ford and Carter administrations. But the initial ideas of the Reagan team involved relatively uncharted areas where available information as well as theory is much thinner.

For example, administration planners generally agreed that limits on nuclear "launchers" (such as missiles or aircraft) were no longer sufficient. But should the new basis for limitations be explosive power (megatonnage), rocket lifting power (throw weight), numbers of individual warheads, or a combination of all three? Answers are very complicated.

Similarly, verification by "national technical means"--that is, inspection by spy satellites and radars--was said to be no longer sufficient. But if so, what proposals for "cooperative measures" can pass muster with the U.S. military, to say nothing of the Soviet military?

Deadlines for studies of these and other points of a negotiation position were set for last fall, but the basic issues are still under debate.

Preparations for the U.S.-Soviet negotiations about medium-range nuclear missiles in Europe, which took priority among arms control policy makers for most of 1981. The bureaucratic spotlight turned to the strategic area only after the Euromissile talks began Nov. 30, an official said.

At the White House yesterday, Paul Nitze, chief U.S. negotiator in the Euromissile talks, reported "substantial progress in dealing with a series of secondary issues" but none in dealing with the central issue. Eugene Rostow, administrator of the Arms Control and Disarmament Agency, seemed more pessimistic, saying that so far "it's not been a serious negotiation." Finally, the Soviet-backed martial-law crackdown in Poland Dec. 13.

U.S. plans had been to use the meeting of Secretary of State Alexander M. Haig Jr. and Soviet Foreign Minister Andrei Gromyko in late January to agree on a date for beginning U.S.-Soviet strategic arms negotiations. But this was deemed politically unwise in the wake of the Polish events.

Two weeks ago, eight different approaches to a U.S. negotiating position were being debated, according to an involved official. These have been reduced to "two or three basic options," he said.

One of the competing proposals turned up in remarkably accurate form two weeks ago in a Newsweek story by David C. Martin, according to officials. This called for U.S. and Soviet ceilings of 8,000 warheads each, of which 4,000 could be on land-based intercontinental missiles, as well as ceilings of 5.9 million pounds of rocket "throw weight" on each side.

With the groundwork nearing completion, the time to decide is approaching--"a most interesting time," an official said.