SDI -- the Strategic Defense Initiative announced by President Reagan in 1983 -- is one issue that separates the presidential candidates of the two parties. The Republicans say they're for it, the Democrats say they're against. But there are differences of nuance and detail among the candidates that could end up making a difference in the next administration.

For the Republicans SDI is a theological issue: the candidates are trying not so much to persuade those who are ignorant or uncertain as they are to prove to true believers that their faith is stronger and purer than any rival's. So they argue about who loves space defense most -- though, fortunately, they do not succumb to the temptation which has afflicted the president of describing SDI as a leak-proof defense against every missile aimed at the United States.

Alexander Haig favors research on SDI, but thinks that full protection of populations is at least 15 years away. Pat Robertson favors SDI, but does not have a fully detailed position yet. George Bush says we must "shift away from offensive retaliation toward greater reliance on strategic defenses," and "must resist the anti-intellectual temptation to cut off the research, development and testing." But he admits "we don't yet have all the answers" -- i.e., we don't know whether space defense can ever work. That leaves him vulnerable to Jack Kemp, who has faith that it can, and urges immediate testing and deployment, which the administration has so far not advocated. Bob Dole has led the Republicans' fight in the Senate to keep the Democrats from barring the administration from testing without further congressional approval. But he does not gush with praise for it as readily as Mr. Kemp or Pete du Pont.

They speak with an optimism about SDI technology that is attractive but has not yet been vindicated by results. You get the sense that Jack Kemp and Pete du Pont have a faith in SDI which will lead them to emphasize it in a general election and push for it in an administration more than George Bush or Bob Dole would. Any Republican administration will probably move ahead at least with measured research, which is a good idea; the questions are whether they would go ahead with testing and deployment, the case for which is anything but compelling, and whether they would be willing to include space defense in a negotiation. Campaign rhetoric does not give completely reliable answers to these questions, but on the difference in degree of passion about the program suggests who would be more and who less likely to use SDI for bargaining purposes.