WASHINGTON -- Senior Obama administration officials have agreed that the number of nuclear warheads the U.S. military deploys could be cut by at least a third without harming national security, according to those involved in the deliberations. Such a reduction would open the door to billions of dollars in military savings, which might ease the federal budget deficit. It also would improve prospects for a new arms deal with Russia before President Barack Obama leaves office, those involved said, but it’s likely to draw fire from conservatives, if previous debate on the issue is any guide.