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

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,” said Sen. Robert Byrd, D-W.V., chairman of the Appropriations Committee. He later added: “It’s time to bring our troops home from Iraq.”

The $124.2 billion bill requires troop withdrawals to begin Oct. 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, Gen. 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. Bush said he will veto the bill so long as it contains a timetable on Iraq, as well as $20 billion in spending added by Democrats.

“The solution is simple: Take out the surrender date, take out the pork, and get the funds to our troops,” said Republican Leader Mitch McConnell, R-Ky.

Sen. Joseph Lieberman, I-Conn., sided with Republicans in opposing the bill.

“We delude ourselves if we think we can wave a legislative wand and suddenly our troops in the field will be able to distinguish between al-Qaida terrorism or sectarian violence. Or that Iraqis will suddenly settle their political differences because our troops are leaving,” Lieberman said.

Democrats said the bill was on track to arrive on the president’s desk by Tuesday, the anniversary of Bush’s announcement aboard the deck of the USS Abraham Lincoln that major combat operations in Iraq had ended.

“The battle of Iraq is one victory in a war on terror that began on Sept. 11, 2001, and still goes on,” Bush said on May 1, 2003, in front of a huge “Mission Accomplished” banner.

Bush since has acknowledged that the war has not progressed as he had hoped. After the November elections in which Democrats swept up enough seats to take the majority, he announced a new strategy that involved sending additional forces to Iraq.

White House spokeswoman Dana Perino said that if Democratic lawmakers timed the sending of the bill to the anniversary of Bush’s speech, it would be “a ridiculous P.R. stunt.”

“That is the height of cynicism, and absolutely so unfortunate for the men and women in uniform and their families who are watching the debate,” she said Thursday morning.

As Democrats pushed through the bill, Petraeus depicted the situation in Iraq as “exceedingly complex and very tough.” He said there have been some improvements in the two months since Bush’s troop buildup began, but “there is vastly more work to be done across the board. … We are just getting started with the new effort.”

Asked at a Pentagon news conference Thursday about the impact on the effort in Iraq if that legislation passed, Petraeus said, “I have tried to stay clear of the political minefields of various legislative proposals.”

In the House, two Republicans _ Reps. Wayne Gilchrest of Maryland and Walter Jones of North Carolina _ joined 216 Democrats in passing the bill. Voting no were 195 Republicans and 13 Democrats.

Rep. John Murtha, D-Pa., said Democrats were still considering what to do after Bush’s veto. One option would be funding the war through September as Bush wants but setting benchmarks that the Iraqi government must meet, he said.

Murtha chairs the House panel that oversees military funding.

“I think everything that passes will have some sort of condition (placed) on it,” he said. Ultimately, Murtha added, the 2008 military budget considered by Congress in June “is where you’ll see the real battle,” he said.

Sen. Joseph Biden, D-Del., said the immediate focus should stay on the president “making such a tragic mistake in vetoing this.” Eventually, “I think he’s going to have to accept constraint on his bad judgment here. . . . We’ve got to keep relentlessly putting pressure on him.”

The nonpartisan Congressional Research Service has said the Army has enough bookkeeping flexibility to pay for operations in Iraq well into July. Lawmakers and Capitol Hill staff aides view mid- to late May as the deadline for completing the war spending bill to avoid hardships.