WASHINGTON -- The Senate approved a $124 billion Iraq spending bill yesterday that would force troop withdrawals to begin as early as July 1, inviting President Bush's veto even as party leaders and the White House launch talks to resolve their differences.

The 51-46 vote was a triumph for Democrats, who just weeks ago had worried about the political wisdom of a veto showdown with the commander in chief as troops fight on the battlefield. But Democrats are hesitant no more. And now that withdrawal language has passed both houses of Congress, even Republicans acknowledge that Mr. Bush won't get the spending bill that he has demanded, with no strings attached.

Mr. Bush is expected to veto the bill early next week. But bipartisan negotiations have already started on a compromise to cool the red-hot war debate, at least on the funding front.

Senate Minority Leader Mitch McConnell, R-Ky., spoke with Mr. Bush yesterday morning and later held initial talks with Senate Majority Leader Harry Reid, D-Nev. Senior Democratic and Republican lawmakers began to weigh alternatives to the legislation's most contentious provision, the binding withdrawal terms. The goal is to be more flexible but still restrain how Mr. Bush conducts the war.

Sen. John Warner, R-Va., who has criticized Mr. Bush's war policy but opposed the Democratic bill as too heavy-handed, singled out one development that has stoked a more cooperative spirit on Capitol Hill: Word that the Iraqi parliament may recess for two months this summer. "That would send a very bad signal to the world that they don't have the resolve that matches the resolve of the brave troops that are fighting in the battle today," he said.

The provision most likely to survive the next round is a set of political and diplomatic benchmarks for the Iraqi government. The language all but certain to be dropped, or at least diluted, would require troop withdrawals to begin as early as July 1 and no later than Oct. 1. Another sticking point is the bill's $21 billion in domestic spending, which Mr. Bush and some Republicans have described as pork.

A significant number of Republicans support the benchmarks -- possibly enough to override a second veto, should Mr. Bush resort to that. They would prod Baghdad officials to build up military forces, crack down on militias and sectarian violence, protect minority rights and manage Iraq's extensive petroleum reserves.

Mr. Bush announced the benchmarks in January in a televised address but set forth no consequences if Iraqis failed to deliver. The approved spending bill would make a continued U.S. troop commitment contingent on Iraqi progress -- although only up to a point. On July 1, if Mr. Bush decides the Iraqis are falling short, U.S. combat forces would be withdrawn over six months. If the government shows progress, the window would be extended until Oct. 1, with troops leaving by March 2008.

GOP leaders signaled yesterday that they are ready to negotiate. In the House, which passed the measure late Wednesday largely along party lines, Minority Leader John Boehner, R-Ohio, said the veto "will give us a chance to sit down with our colleagues across the aisle and find common ground."

While a deadline for bringing the troops home would not survive a veto, the Democrats' legislative victory is significant, beating expectations on both sides of the aisle. But it also opens the door to complicated new challenges. House and Senate Democratic leaders must establish terms that are tough enough to satisfy a large antiwar faction, particularly in the House. At the same time, they must water down the bill to a point where Mr. Bush will sign it.

Mr. Bush, Vice President Dick Cheney and other administration officials accuse Democrats of crass political posturing.