THE Strategic Defense Initiative has become still another dubious litmus test - in this case for indicating which presidential candidate is strongest on national defense. The GOP platform commits the party to ``rapid and certain deployment of SDI as technologies permit.'' SDI enthusiasts say technologies already exist that would permit deployment in a limited way. Last week George Bush said he backed funding for SDI research at levels President Reagan has sought. But he indicated he would defer a decision to deploy until ``all the cards are on the table.''

Mr. Bush would now entertain a limited role for a deployed defense; in July he said he wouldn't. He also suggests that cost could affect deployment.

Gov. Michael Dukakis condemns ``star wars,'' as SDI is widely known, as ``...a fantasy, a fraud, and we ought to stop spending billions for it.'' Actually, he does favor research on strategic defense, but at its pre-1983 level. At that time funding was about $1 billion a year.

And which vision of strategic defense is he criticizing? The President's leakproof space-based umbrella, in which case his assessment is probably right. But that says nothing about his views on a more limited system. After all, the Antiballistic Missile Treaty of 1972 allows the United States to field ground-based defenses to protect one target; currently it defends none.

The SDI dispute shows a pragmatic Republican and a flexible Democrat arguing how best to spend finite sums of money to maintain national security.

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