In a statement, the administration said it would "strongly oppose any amendment to weaken, delay or repeal" the base-closing process and Defense Secretary Donald H. Rumsfeld would recommend that the president veto any bill that includes such a provision.
Republican Sen. John Thune of South Dakota, who is trying to save Ellsworth Air Force Base, late Thursday proposed an amendment to the defense bill that would require the Pentagon to complete several operational reviews and return U.S. troops from Iraq before Congress signs off on the final version of the base-closing plan.
Thune's amendment has the support of Republicans and Democrats from Maine, Connecticut, New Jersey and New Mexico.
The Senate debated the bill Friday but may not complete work on it until September.
It's unclear whether Senate leaders and top members of the Senate Armed Services Committee who oppose the provision, GOP Sens. John Warner of Virginia and John McCain of Arizona and Democratic Sen. Carl Levin of Michigan, have the votes to defeat it.