Just two months ago, Salvadorans, Guatemalans and other refugees from Central America were ecstatic. President Clinton had threatened to veto a final budget deal unless more than a million immigrants, including them, were allowed a chance at becoming legal residents. But in the wake of elections that favored Republicans, and facing fierce GOP opposition to broader proposals, Clinton has dropped his threat and agreed to an immigration package last month that helps just over half as many people. In the end, about 400,000 immigrants from Salvador, Guatemala and Honduras, plus 50,000 or so from Haiti and Liberia, find themselves with little reasonable chance at becoming legal U.S. residents, let alone citizens. Many, including tens of thousands in the Washington area, are likely to face new threats of deportation after more than a decade of living in the United States.