WASHINGTON - Defying a veto threat, the Democratic-controlled Senate narrowly signalled 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 (George W.) Bush that the time has come to find a new way forward in this intractable war.''

But Republicans and Senator 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 antiwar candidate.

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.

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

Additionally, Senator Chuck Hagel of Nebraska, a vocal critic of the war, sided with the Democrats, assuring them of the majority they needed to turn back a challenge led by Senator Thad Cochran (R-Miss.)

The debate came on legislation that provides US$122 billion to fund the wars in Iraq and Afghanistan as well as domestic priorities such relief to hurricane victims and payments to farmers.

The House has already passed legislation requiring troops to be withdrawn by Sept. 1, 2008. 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.

"Frankly, I think we'd like to reach out to the president ... and say, 'Mr. President, this is not a unilateral government. It is a separation of powers, and the Congress of the United States is assuming review,''' House Majority Leader Steny Hoyer of Maryland told reporters as the Senate debated the war.