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 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 U.S. troops. It came days after the House approved a binding withdrawal deadline of Sept. 1, 2008, and increased the likelihood of a veto confrontation this spring.

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.”

“It is a choice between staying the course in Iraq or changing the course in Iraq,” he said.

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

Sen. John McCain, R-Ariz., a presidential hopeful, said “we are starting to turn things around” in the Iraq war, and added that critics “conceive no failure as worse than remaining in Iraq and no success worthy of additional sacrifice. They are wrong.”

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

“The president’s strategy is taking America deeper and deeper into this quagmire with no exit strategy,” said Hagel, the most vocal Republican critic of the war in Congress.

Vice President Dick Cheney traveled to the Capitol in case his vote was needed to break a tie, a measure of the importance the administration places on the issue.

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 Thursday.