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.
Base closings "should not occur while this country is engaged in a major war," said Thune.
Warner said the provision will "hold in limbo" the entire base-closing process. "You'd put a cloud of indecision and doubt over all the communities that will be affected," Warner said.
Thune's base-closing amendment is not the only one that could jeopardize the defense bill.
In its statement outlining its positions on the bill, the administration also said White House advisers would recommend a veto if provisions are included regulating the treatment of terror suspects in U.S. custody or establishing a commission to review their handling.
Such amendments would "interfere with the protection of Americans from terrorism by diverting resources from the war to answer unnecessary or duplicative inquiry or by restricting the president's ability to conduct the war effectively," the administration says.
Republicans, including McCain and Sen. Lindsey Graham of South Carolina, plan to offer amendments about the treatment and rights of detainees at the Guantanamo Bay prison camp. And Democrats plan an amendment that would set up an independent commission to review claims of U.S. abuse of prisoners at the camp or elsewhere.
"If they have done nothing wrong, and have nothing to hide, they should welcome this amendment," Sen. Edward Kennedy, D-Mass., said Friday.
Rumsfeld released his sweeping proposal in May to close or shrink 62 major domestic bases and hundreds of smaller installations to save money and streamline services.
The commission reviewing the proposal must send it to the president, with revisions if necessary, by September for his approval. It then goes to Congress, which must accept it or reject it in its entirety.