The annual defense bill - previously approved by a Senate committee - does not usually lead to political deadlock. It must be hammered out to ensure salaries for soldiers and the defense forces' construction and industrial investments.
The Senate Minority Leader, Democrat Chuck Schumer, justifies his opposition by saying that he does not want to contribute to indirectly approving "the recklessness we see unfolding in Iran."
Republicans need 60 votes to pass, but only reached 50 in a vote on Tuesday.
It will probably be difficult to achieve this week, says Democratic Senator Tim Kaine.
