By Emelie Rutherford

The Senate was prepared to pass a $636.3 billion defense spending bill for fiscal year 2010 last night, as lawmakers waited to learn if the actual vote would take place on Saturday or sooner.

The Senate was expected to grant final congressional approval to the Pentagon appropriations legislation that cleared the House via a 395-34 vote on Wednesday. Yet the timing of the actual Senate vote has been in question while Democrats and Republicans debate the massive healthcare-reform bill.

To expedite the defense bill’s passage, Senate Majority Leader Harry Reid (D-Nev.) filed a cloture motion Wednesday night, a procedural move that forces a vote. Unless Republicans agreed to take up the defense bill sooner, Senate Democrats planned yesterday to hold a cloture vote around 1 a.m. today followed by a vote to actually pass the defense measure at approximately 7 a.m. tomorrow.

If the defense bill is passed tomorrow, it will soon after go to the White House for President Barack Obama’s signature.

The timing of the vote is sensitive because FY ’10 began Oct. 1 and the Defense Department has been operating under a temporary funding resolution that expires after today. As a backup plan, lawmakers have prepared another so-called continuing resolution for the Pentagon that would extend until Dec. 23.

Yet that second resolution may not be needed. Peter Orszag, director of the White House’s Office of Management and Budget, said while the current resolution expires at midnight tonight, if the defense bill is signed into law by midnight tomorrow the new act will provide funding for the Defense Department for the entire day tomorrow.

Orszag’s explanation, in Dec. 16 letters to Reid and House Speaker Nancy Pelosi (D-Calif.), suggests Obama would be prepared to sign the defense bill tomorrow.

Obama is not expected to veto the legislation, even though it includes $465 million in unrequested funding for developing the F-35 Joint Strike Fighter’s second engine, developed by General Electric [GE] and Rolls-Royce, which he and Defense Secretary Robert Gates do not want to continue.

The appropriations measure also funds 10 of Boeing‘s [BA] C-17 cargo aircraft that the Pentagon did not want. Other unrequested items include $45 million to recoup research and development investments already made in the canceled Lockheed Martin [LMT] VH-71 presidential helicopter program, and funding to buy 18 of Boeing’s F/A-18E/F Super Hornet fighter jets, instead of the nine requested.