WASHINGTON — A defiant, Democratic-controlled Senate approved legislation Thursday calling for the withdrawal of U.S. combat troops from Iraq within a year, propelling Congress closer to an epic, wartime veto confrontation with President Bush.

The 51-47 vote was largely along party lines, and like House passage of a separate, more sweeping challenge to the administration’s war policies a week ago, fell far short of the two-thirds margin needed to overturn the president’s threatened veto. It came not long after Bush and House Republicans made a show of unity at the White House.

“With passage of this bill, the Senate sends a clear message to the president that we must take the war in Iraq in a new direction. Setting a goal for getting most of our troops out of Iraq is not — not, not — cutting and running,” said Sen. Robert C. Byrd, D-W.Va., shortly before the vote. Passage cleared the way for negotiations on a compromise with the House.

Senate Republican leader Mitch McConnell of Kentucky rebutted quickly. “Nothing good can come from this bill,” he said. “It’s loaded with pork that has no relation to our efforts in Iraq and Afghanistan, and it includes a deadline for evacuation that amounts to sending a ‘Save the Date’ card to al-Qaida.”

Several blocks away, the commander in chief stood with Republican House members and told reporters they were united. “We expect there to be no strings on our commanders and that we expect the Congress to be wise about how they spend the people’s money,” he said.

In private, Bush was at least as emphatic, according to participants at a closed-door session in the White House East Room with the GOP rank and file. “He said he will veto a bill that comes to his desk with too many strings attached or too much spending,” said one official in attendance, speaking on condition of anonymity because the meeting was closed to the press.

While both sides have spoken positively about a need for compromise, there also was strong political pressure within both parties for a veto fight.

Democrats are under pressure to challenge Bush on the war following their victories in last fall’s elections. At the same time, Republicans say Bush will blame anti-war lawmakers if money for troops begins to run short and will accuse them in any event for ceding ground to the terrorists in the Middle East.

Gen. Peter Pace, chairman of the Joint Chiefs of Staff, told the House Defense Appropriations Committee during the day that a delay in funding would have a chain reaction that could keep units in Iraq longer than planned. He said if the bill is not passed by May 15, the Army will have to cut back on reserve training and equipment repairs.

The House-passed measure requires the withdrawal of combat troops by Sept. 1, 2008. The Senate bill mandates the beginning of a withdrawal within 120 days, and sets a nonbinding goal of March 2008, for its completion.