WASHINGTON – The Democratic-controlled Senate narrowly signaled support Tuesday for the withdrawal of U.S. combat troops from Iraq by next March, triggering an instant veto threat from the White House in a deepening dispute between Congress and commander in chief.

Republican attempts to scuttle the non-binding timeline failed, 50-48, largely along party lines.

The vote 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. It came days after the House approved a binding withdrawal deadline of Sept. 1, 2008.

After weeks of setbacks on the Senate floor, 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, “we are starting to turn things around” in the Iraq war, and added that a timeline for withdrawal would encourage terrorists in Iraq and elsewhere.

Bush had previously said he would veto any bill that he deemed an attempt to micromanage the war, and the White House freshened the threat a few hours before the vote – and again afterward. “The president is disappointed that the Senate continues down a path with a bill that he will veto and has no chance of becoming law,” it said.

Similar legislation drew only 48 votes in the Senate earlier this month, but Democratic leaders made a change that persuaded Nebraska’s Democratic Sen. Ben Nelson to swing behind the measure.

Additionally, Sens. Chuck Hagel of Nebraska and Gordon Smith of Oregon, vocal critics of the war, sided with the Democrats, assuring them of the majority they needed to turn back a challenge led by Sen. Thad Cochran, R-Miss.

The debate came on legislation that provides $122 billion to fund the wars in Iraq and Afghanistan as well as domestic priorities such relief to hurricane victims and payments to farmers. Final passage is expected Wednesday or Thursday.

Separately, supporters of an increase in the minimum wage readied an effort to attach the measure to the spending bill, along with companion tax cuts that Republicans have demanded. The House and Senate have passed different versions of the bill but have yet to reach a compromise. The Senate vote assured that the Democratic-controlled Congress would send Bush legislation later this spring that calls for a change in war policy. A veto is a certainty, presuming the president follows through.

That would put the onus back on the Democrats, who would have to decide how long they wanted to extend the test of wills in the face of what are likely to be increasingly urgent statements from the administration that the money is needed for troops in the war zone.

“I hope he will work with us so we can come up with something agreeable for both” sides, Reid said at a post-vote news conference. “But I’m not anxious to strip anything out of the bill.”

As drafted, the legislation called for troop withdrawal to begin within 120 days, with a non-binding goal that calls for the combat troops to be gone within a year.

The measure also includes 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 – provisions designed to attract support from Nelson and Sen. Mark Pryor of Arkansas.

Despite the change, Pryor voted to delete the timeline.

The vote was a critical test for Reid and the new Democratic majority in the Senate nearly three months after they took power. Despite several attempts, they had yet to win approval for any legislation challenging Bush’s policies.

Republicans prevented debate over the winter on non-binding measures critical of Bush’s decision to deploy an additional 21,500 troops. That led to the 50-48 vote derailing a bill that called for a troop withdrawal to begin within 120 days.