It was only a victory of two votes — but the two votes were enough to change the political equation in Washington:

Defying a veto threat, the Democratic-controlled Senate narrowly signaled support Tuesday for the withdrawal of U.S. combat troops from Iraq by next March.

Republican attempts to scuttle the non-binding timeline failed on a vote of 50-48, largely along party lines. The roll call marked the Senate’s most forceful challenge to date of the administration’s handling of a war that has claimed the lives of more than 3,200 U.S. troops.

Three months after Democrats took power in Congress, Senate Majority Leader Harry Reid said the moment was at hand to “send a message to President Bush that the time has come to find a new way forward in this intractable war.”

But Republicans—and Sen. Joseph Lieberman, an independent Democrat—argued otherwise.

John McCain, R-Ariz., a presidential hopeful, said that “we are starting to turn things around” in the Iraq war” and that a timeline for withdrawal would embolden the terrorists in Iraq and elsewhere.

The effect of the timeline would be to “snatch defeat from the jaws of progress in Iraq,” agreed Lieberman, who won a new term last fall in a three-way race after losing the Democratic nomination to an anti-war insurgent.

Bush had previously said he would veto any bill containing the timeline, and the White House freshened the threat a few hours before the vote on Tuesday. “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,” it said in a statement.

The change will likely be more qualitative than quantitative: the Democrats clearly do not have the votes to override a Bush veto.

But by passing the resolutions in the House and Senate, the Democrats have (narrowly) delivered on the party’s progressive base’s expectations and also likely got thumbs-up from some Centrist Democrats and independents who seek greater oversight on the war or some sign that the war is not open-ended or conducted via a blank check. Reuters adds:

Bush has promised to veto any legislation with dates for withdrawing from Iraq, including a measure passed by the U.S. House of Representatives on Friday.

“This war is not worth the spilling of another drop of American blood,” Senate Majority Leader Harry Reid, a Nevada Democrat, said in pleading for the troop withdrawal plan included in the money bill.

The Senate vote came four days after the House passed its version of a war-spending bill that would set a mandatory September 1, 2008, deadline for withdrawing all U.S. combat troops from Iraq.

Under the Senate bill, which is still being debated, the United States would begin a phased withdrawal of troops this year with the goal, not the requirement, that it be completed by March 31, 2008.

Tuesday’s vote in the Senate marked progress for Democrats, who failed recently to pass a similar, non-binding resolution calling for a troop withdrawal.

One key question that is likely to continue to come up will be the the plans of Joe Lieberman, who some progressives think is toying with switching parties if the Democratic majority takes too strong an anti-war stance. But that seems unlikely, given continued erosion shown in polls for the war, Bush’s low polling status, and the fact that Lieberman differs from the Republicans on a host of other issues. And, if he switched parties, he’d lose any future bargaining power with the on-the-ascent Democrats.

UPDATE: The Los Angeles Times adds this:

A conference committee of senators and House members will have to write compromise legislation that can clear both chambers and be sent to the White House.

GOP Senate leaders were unable today to rally the votes to remove the withdrawal requirement from the measure and unwilling to use parliamentary tactics to block it.

Two Republicans crossed the aisle and voted with the majority Democrats, and one Democrat and one independent voted with the Republicans.

Now, with the Senate poised to approve the war-funding bill that includes the withdrawal timeline as soon as Wednesday, congressional Democrats are about to present Bush with a stark choice: veto a bill funding the war or deal more directly with war critics than at any point since the invasion four years ago.

“He doesn’t get everything he wants now, so I think it’s time that he started working with us,” said Senate Majority Leader Harry Reid (D-Nev.), a lead architect of the Democratic campaign to escalate pressure on the president to change his war policy.

The fact that both chambers attached the withdrawal timetable to legislation to pay for the war effort complicated the strategy on both sides. The Pentagon has warned that unless a funding bill is approved by Congress, and signed by the president in the next month, military deployments to Iraq could be jeopardized.

Neither the Democratic Congress nor Bush wants to be seen by the public as responsible for delaying much-needed supplies to the troops on the ground.

Is a NEW political game of chicken about to begin?