WASHINGTON — The Democratic-controlled Senate narrowly signaled support yesterday for the withdrawal of American 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 nonbinding 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 American troops. It came days after the House approved a binding withdrawal deadline of September 1, 2008.

After weeks of setbacks on the Senate floor, the majority leader, Senator 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 Senator Lieberman, an independent Democrat — argued otherwise.

Senator McCain, a Republican of Arizona and 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.

Mr. 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 Senator Nelson, a Democrat of Nebraska, to swing behind the measure.

Additionally, Senators Hagel of Nebraska and 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 Senator Cochran, a Republican of Mississippi.

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 today or tomorrow.

Meanwhile in Iraq, two truck bombs shattered markets in Tal Afar yesterday, killing at least 63 people and wounding dozens in the second assault in four days on a predominantly Shiite Muslim city hit by a resurgence in violence a year after it was held up as a symbol of U.S. success.