IF THE REPUBLICANS vie to prove their faith in "SDI," the Democratic presidential candidates vie to prove their lack of it in "Star Wars." Speaking (especially in dovish Iowa) to audiences deeply suspicious of every Pentagon initiative, they heap ridicule on space defense, seldom bothering to make the serious arguments that underlie some of their jibes. "Our children," says Michael Dukakis, "don't deserve wars in space; they deserve peace on earth." Joseph Biden proclaims that "the next president will preside over Star Wars or an arms control agreement." The Republicans argue with some merit that it was the prospect of SDI that got the Soviets to the bargaining table.

"After Star Wars," said Bruce Babbitt in his debate with Pete du Pont, "is a sequel called 'The Empire Strikes Back.' " It doesn't make sense to build an Astrodome, says Richard Gephardt. Mr. Babbitt believes any gain in defense can be overcome by Soviet advances in offensive weapons; Mr. Gephardt believes that SDI would be so destabilizing it shouldn't be deployed even if it works, though he doubts it will, and that SDI spending could distort the entire defense budget; Albert Gore thinks it's destabilizing, is against all testing and believes that it could be bargained away for useful concessions from the Soviets; Jesse Jackson is against testing and deployment, opposes research that would have military application; Paul Simon thinks the idea is "fatally flawed."

Interestingly, none of the Democrats carries his scorn for this program to the point of shutting down research altogether. And the amount of research the Democrats talk about is not negligible. The numbers are these: the administration recommended $5.7 billion for 1988, the Senate Armed Services Committee has voted $4.5 billion, and the House voted $3.1 billion. No one can say what such a program will produce or should cost, but our sense is that the House and the Senate are near the right place and that most of the Democratic presidential candidates are not far off.

That leaves the Democrats in a fairly reasonable place on SDI: skeptical of the boosters, properly worried about destabilization, but willing to fund some considerable research and ready to bargain in return for useful concessions from the Soviets. Though you wouldn't know it from the rhetoric, that's not too far from where some of the Republicans end up on the issue too.