WASHINGTON — A defiant Democratic-controlled Senate passed legislation yesterday that would require the start of troop withdrawals from Iraq by October 1, propelling Congress toward a historic veto showdown with President Bush on the war.

At the White House, the president immediately promised a veto.

“It is amazing that legislation urgently needed to fund our troops took 80 days to make its way around the Capitol. But that’s where we are,” said deputy press secretary Dana Perino.

The 51–46 vote was largely along party lines, and like House passage of the same bill a day earlier, fell far short of the two-thirds margin needed to overturn the president’s threatened veto. Nevertheless, the legislation is the first binding challenge on the war that Democrats have managed to send to Bush since they reclaimed control of both houses of Congress in January.

“The president has failed in his mission to bring peace and stability to the people of Iraq,” Senator Byrd, a Democrat of West Virginia and chairman of the Appropriations Committee, said. He later added: “It’s time to bring our troops home from Iraq.”

The $124.2 billion bill requires troop withdrawals to begin October 1, or sooner if the Iraqi government does not meet certain benchmarks. The House passed the measure Wednesday by a 218–208 vote.

Across the Potomac River at the Pentagon, the top U.S.commander in Iraq, General David Petraeus, told reporters the war effort likely will “get harder before it gets easier.”

Republicans said the vote amounted to little more than political theater because the bill would be dead on arrival after reaching the White House. Mr. Bush said he would veto the bill so long as it contains a timetable on Iraq, as well as $20 billion in spending added by Democrats.