The Senate Appropriations Committee approved a $121.7 billion supplemental war spending measure Thursday, sidestepping a lengthy debate over provisions requiring withdrawal of troops from Iraq starting 120 days after enactment.

The panel approved the measure with the Iraq provisions by voice vote, with Republicans vowing to make their push to remove the provisions on the Senate floor. The bill could be considered on the floor as early as March 26.

Meanwhile, House Democratic leaders appeared confident that they had rounded up enough votes to pass that chamber’s $124.3 billion war supplemental measure (HR 1591), which also includes Iraq withdrawal provisions. The House floor debate began Thursday.

The House and Senate bills exceed President Bush’s $103 billion request for war spending and hurricane relief, with both chambers adding billions of dollars for domestic priorities such as agriculture disaster relief and a children’s health insurance program.

The White House has threatened to veto the House measure because of its Iraq provisions and the extra spending. Rob Portman, the director of the Office of Management and Budget, expressed similar concerns about the Senate measure Thursday.

“We strongly urge both the House and the Senate to give U.S. military commanders the flexibility and resources they need to succeed, and eliminate the unrelated spending from the bill,” Portman said.

The House is expected to take a final vote on the supplemental Friday; top Democrats were confident Thursday afternoon that they had the 218 votes needed to win passage. “We are there,” a senior Democratic aide said.

The Senate panel’s decision to approve the bill by voice vote was a senatorial courtesy to the absent Tim Johnson, D-S.D., who is recuperating from a brain hemorrhage.With the panel split 15-14, Johnson’s absence would have led to a tie vote and failure of the measure if members had voted along party lines by roll call vote. Members are allowed to vote by proxy on amendments but not on the bill itself.

In the interest of moving the bill to the floor expeditiously, the top members of the committee, Chairman Robert C. Byrd, D-W.Va., and ranking Republican Thad Cochran of Mississippi, encouraged members to withhold amendments until the bill reached the Senate floor.

Despite their urgings, Alabama Republican Richard C. Shelby introduced an amendment to strike the Iraq provisions, which are comparable to a resolution offered by Majority Leader Harry Reid, D-Nev., that failed on the floor, 48-50. The provisions require the start of a troop withdrawal from Iraq and set a goal for completing the process by March 2008.

The legislation also includes benchmarks for progress in Iraq and a requirement of regular reports from commanders. Those were sought by Democrat Ben Nelson of Nebraska, who had voted against Reid’s resolution on the Senate floor March 15.

At the request of Ted Stevens, R-Alaska, who invoked Johnson’s absence and said there was an agreement to postpone the debate over the Iraq provisions until the floor, Shelby withdrew his amendment.

After the markup, Shelby said, “This is an ongoing battle. We had to raise it.”

Cochran announced at the outset of the markup that he will seek to strike the Iraq conditions when the bill advances to the floor. He also noted that the administration had said the war spending was needed by the end of April, and that he believed including the Iraq language “will only slow down the bill and invite a presidential veto.”

Johnson is not expected to be available next week for what could be a close floor vote on the supplemental spending bill, said his spokeswoman, Julianne Fisher.

The Senate panel did adopt a number of relatively non-controversial amendments, mostly by voice vote, including one by Patrick J. Leahy, D-Vt., to extend a milk-price subsidy program by one month, through Sept. 30, 2007. That extension would cost $31 million.

The House supplemental bill, meanwhile, includes funding for military operations in Iraq and Afghanistan, plus billions of dollars for non-war spending. It would require the withdrawal of all but a vestige of the current U.S. force in Iraq by August 2008. It would also set minimum readiness standards for forces before they deployed, although the president could waive those requirements.

Speaker Nancy Pelosi, D-Calif., personally met on the House floor Thursday with wavering members to capture their votes. During one vote she sat for a while with Donald M. Payne, D-N.J., her chin resting in her hands as she listened to his views.

With almost every House Republican expected to vote against the measure, Pelosi’s challenge was to sway liberals who think the war provisions would not bring U.S. troops home soon enough, and yet hold on to the votes of centrists who oppose the strict guidelines for the military and the additional domestic spending.

Her efforts appeared to succeed, as Democrats predicted victory. “I think the vote will carry,” said Sanford D. Bishop Jr. of Georgia, a member of the whip team.

The key to Pelosi’s success appeared to be her inroads with progressives. Four House Democrats from California who led the anti-war group — Lynn Woolsey, Barbara Lee, Diane Watson and Maxine Waters — in a statement signaled that enough of the bloc’s members would vote for the supplemental for the legislation to pass.

Waters, head of the roughly 75-member Out of Iraq Caucus, will vote against the bill but said she had “released” an unspecified number of caucus members to vote for it after they had expressed concerns that a “no” vote would bring reprisals from leaders, undermine Pelosi in a major test and threaten Democratic unity. “We’re not trying to stop them from voting [‘yes’] if they feel in the final analysis they won’t be comfortable for the rest of their careers with this vote,” Waters said.

Several liberal House Democrats said Thursday that they would vote for the bill, including Barney Frank of Massachusetts, Jim McDermott of Washington, Tom Allen of Maine and Hank Johnson of Georgia.

McDermott predicted up to 240 votes for passage, saying members from both parties would vote for the supplemental because they do not want to be responsible for defeating it. “There are a lot of people who do not want to be the one who pushed it over the line,” he said.

One liberal Democrat, Jim McGovern of Massachusetts, said he was voting for the bill, calling it “the toughest measure that we can get passed and get 218 votes for.”

In the centrist Democrats’ camp, Jim Cooper of Tennessee, who had not publicly stated his decision before, said he would “probably” vote for the measure, despite his disapproval of its unrequested spending and the strict rules for the military.

GOP opposition to the bill remained solid.

“They’re trying to buy support for this dangerous proposal by loading it up with unnecessary spending,” said Minority Leader John A. Boehner of Ohio. “If Democrats were able to vote their conscience, this bill would have no chance of passing.”

At least 10 moderate Republicans signed a letter to Democratic leaders that criticized the supplemental. The lawmakers said they were “concerned that setting an arbitrary timeline for withdrawal would tie the hands of our commanders in the field and assuredly lead to an impasse with the White House, which in turn will further delay this crucial funding.”

Early Thursday, the House Rules Committee adopted a closed rule for the measure, meaning no amendments will be allowed. The rule allows for a motion to recommit the bill with instructions, which will be Republicans’ only chance to weigh in on the floor. Boehner said Republicans have not decided what, if any, action they would take on a motion to recommit.

The minority party has used the tactic several times this session to alter legislation.

Jonathan Allen, Susan Ferrechio, Patrick Yoest, Catharine Richert and Daphne Retter contributed to this story.