WASHINGTON - An upcoming Senate vote on the Iraq war could come down to just one or two votes, testing Democratic unity on a proposal to begin bringing combat troops home.

Democratic Sens. Mark Pryor and Ben Nelson are expected to deliver the critical votes this week, when members decide whether to uphold legislation that orders some troops home right away, with the goal of ending combat missions by March 31, 2008.

The provision is attached to a $122 billion bill that would fund the wars in Iraq and Afghanistan.

“The United States Senate will ensure they have everything they need to continue this fight as we have done,” said Majority Leader Harry Reid, D-Nev. But, he added, “We must also ensure that our soldiers have a strategy for success.”

The bill is similar to one the House passed last week, but with a tougher deadline. While the Senate identifies March 2008 as a goal - giving the president leeway to ignore the deadline - the House voted 218-212 to require all combat troops out as of Aug. 31, 2008.

The withdrawal proposals have drawn veto threats from President Bush and rallied Republicans against the bill.

“This and other provisions would place freedom and democracy in Iraq at grave risk, embolden our enemies and undercut the administration's plan to develop the Iraqi economy,” the White House said in a statement today.

Sen. Thad Cochran, R-Miss., has proposed striking the withdrawal provision, which GOP members say would broadcast the nation's war plans to the enemy and tie the hands of military commanders.

“It's a bad message all the way around,” said Sen. Jon Kyl, R-Ariz.

Whether Republicans have enough votes to beat the narrow Democratic majority depends upon their ability to entice Democratic defections.

Senate Democrats hold a slim 51-49 majority. And with Sen. Joseph Lieberman, an independent Democrat, supportive of the president's Iraq policy and Democratic Sen. Tim Johnson of South Dakota recuperating from a brain hemorrhage, Democrats this year have been unable to push through legislation critical of the war.

On March 15, the Senate rejected by a 50-48 vote a resolution calling for troops to leave by March 2008. Republican Sen. Gordon Smith of Oregon sided with Democrats in support of the measure, but Nelson of Nebraska and Pryor of Arkansas opposed announcing a timetable for withdrawal.

Since then, Reid and others have altered the legislation in hopes of persuading the two Democrats. The changes include a series of suggested goals for the Iraqi government to meet to provide for its own security, enhance democracy and distribute its oil wealth fairly.

Nelson has since swung behind the bill, contending the benchmarks are necessary to measure progress.

But Republicans hope they can still attract his support because their amendment would eliminate the withdrawal date while retaining the benchmarks Nelson wanted.

Also critical to the upcoming vote is Pryor, who says he would only support a timetable in Iraq if it were classified.

“I think if the public timetable remains, Senator Pryor would likely oppose” the Democratic proposal, said spokesman Michael Teague.

Senate Republican Leader Mitch McConnell, R-Ky., predicted Monday he had the votes to strike the withdrawal language. But even if he fails to keep it out of a final bill - after it is negotiated with the House - McConnell said Republicans won't block final passage because he knows the president will veto it, the sooner the better.

Unable to override Bush's veto, Democrats would have to redraft the bill without a “surrender deadline,” McConnell said.

“We're not interested in letting the political posturing get in the way” of providing resources to the troops, he said.

The legislation also provides about $20 billion in domestic spending and increasingly looks like a magnet for far-flung issues such as a proposed increase in the minimum wage.

Republicans have demanded tax cuts as a condition for their support of a higher minimum wage, and officials said key senators were drafting a provision for debate that would include both those issues. It calls for tax cuts at least as high as the $8.3 billion package the Senate passed earlier, if not larger. House Democrats have labeled that amount excessive.