President Reagan's ballyhooed Strategic Defense Initiative, which he doesn't like calling "Star Wars," has become a ruling passion in his second term.

A few weeks ago, administration officials were making the case for SDI with a wink, hinting broadly in private sessions with reporters that its chief use was as a lever to bring the Soviets back to the arms negotiations table in Geneva. But Reagan has taken every opportunity to insist that he views the SDI as far more than a talking point.

In an interview last week, the president told The New York Times that he would not abandon defensive research even if the Soviets offered to trade offensive weapons. He brushed aside the distinction of protecting missiles and the more difficult task of protecting people from nuclear missiles and said, "We're seeking a nonnuclear weapon that could render these weapons obsolete."

That was Reagan's position March 23, 1983, when he first proposed the SDI in what then seemed almost a by-the-way ending to a nationally televised speech calling for approval of his defense budget. "Tonight, we're launching an effort which holds the promise of changing the course of human history," Reagan said.

Although the president said what he meant and meant what he said, some important administration officials think that events will bring him to the realization that he can have a genuine arms control agreement if he is flexible on "Star Wars."

Their expectation is that the Soviets will make a serious proposal about strategic nuclear arms reductions at Geneva, perhaps one surprisingly acceptable. At this point, the president would be faced with the choice of accepting a proposal that would cast him as a peacemaker at the price of giving up a theoretical system unlikely to produce results for at least a decade.

Some administration officials think that, if this happens, the Great Communicator will be transformed into the Great Negotiator and take what he can get. They say Reagan might be able to have his cake and eat it, too, by signing some treaty protocol that would prohibit deployment of a defensive system -- as the 1972 ABM Treaty does -- but permit limited research. Reagan could then leave office with a treaty to his credit, his beliefs intact and the problem of defensive deployment left to a future president.

Maybe this sophisticated view of Reagan will turn out to be correct, because he does have the capacity to discard cherished ideas in behalf of more realizable goals.

But my reading of Reagan on "Star Wars" is different. I think this is one of those issues on which he thinks that he has seen a future that works. The idea of "mutual assured destruction" as a permanent feature of superpower relationships has long bothered him, and he was quick to latch onto "Star Wars" when deputy national security affairs adviser Robert C. McFarlane recommended it in January 1983.

Although the notion of a defensive shield that could render enemy missiles harmless is only science fiction, the breathtaking simplicity of the idea appeals to Reagan. Whether it could work, he doesn't know, and neither do his critics.

But a case could be made that the "Star Wars" idea has accomplished a great many things, not all of them helpful to Reagan's program. For one, it has forced a debate among Democrats, prodding the party out of power to look at national security in less doctrinaire terms.

The most visible result of this reexamination came when Rep. Les Aspin (D-Wis.), the thoughtful new chairman of the House Armed Services Committee, announced that he is considering support of the SDI.

But Aspin said at the same time that he might abandon support of the MX missile in favor of strategic defense. He gave public voice to a private worry within the administration that "Star Wars" will undermine shaky congressional support for the MX. Those who worry that the SDI might become a cover for a nuclear first strike ought to be even more worried about the MX, especially if it is deployed in vulnerable silos as proposed by the administration.

So the SDI has helped bring the Soviets back to the bargaining table, touched off a vital debate within the opposition and raised new questions about the nation's most controversial missile system. As the president might say, that's not chopped liver.

Reaganism of the Month: Speaking at a White House ceremony commemorating the 75th anniversary of the Boy Scouts, Reagan, 74, said, "I'm delighted to celebrate anything older than I am."