House Democratic leaders remained short of votes Tuesday to pass a war-spending bill mandating a withdrawal of most U.S. forces from Iraq by 2008, raising the possibility that the vote could be delayed.

In the Senate, meanwhile, it was unclear whether the Appropriations Committee, which plans to mark up its version of the supplemental bill March 22, would include a provision endorsing a timetable for ending U.S. involvement in the war. A senior Democrat on the panel said he doubted it would.

House Majority Leader Steny H. Hoyer, D-Md., was optimistic that Democrats, with 233 members, could garner the 218 votes they need to pass the $124.3 billion emergency supplemental (HR 1591) this week. But he acknowledged that they were still short and therefore the floor vote tentatively set for March 22 could be delayed.

Democratic leaders were bolstered Tuesday by new support from liberal groups and from several anti-war House Democrats. Still, faced with united opposition from House Republicans, Democratic leaders struggled to prevent defections on both the left and the right flanks of their caucus.

“If you were to ask me if we have 218 people that I know are definite yeses right this minute, the answer to that is no,” Hoyer said Tuesday. “If you ask me if I think we’ll have 218 on this bill when we call it up for a vote, the answer to that is yes. If you ask me do I think we’ll need to delay it, I hope the answer to that is no, and I believe it’s no.”

The measure exceeds President Bush’s supplemental request by $21.3 billion and contains spending for the wars in Iraq and Afghanistan as well as for such domestic priorities as agriculture disaster relief and children’s health insurance. The most controversial provision would require a withdrawal of most U.S. forces from Iraq as soon as New Year’s Day but no later than the end of August 2008.

With the House vote expected to be so close, every lawmaker counts, none more so than wavering Democratic members of the Out of Iraq Caucus on the left and the “Blue Dog” Democrats on the right.

The Out of Iraq Caucus includes some 75 lawmakers, or roughly one-third of the Democratic Caucus, and Democrats need to win the support of the overwhelming majority of them. Many are wary of a bill they say takes too long to pull U.S. troops from Iraq and does not enforce the withdrawal mandate with funding restrictions.

For example, Maxine Waters of California, a member of the Democratic whip team and leader of the Out of Iraq Caucus, said she will vote against the measure and is recruiting others to do so. Democratic House opponents also include John Lewis of Georgia, Dennis J. Kucinich of Ohio and Pete Stark of California.

But Democratic leaders received a dose of support when several anti-war House Democrats — including Jerrold Nadler of New York, Patrick J. Murphy of Pennsylvania and Jan Schakowsky of Illinois — spoke in favor of the leadership position in a closed-door meeting of Democrats.

“This vote advances the cause of bringing our troops home,” Schakowsky said after Tuesday’s meeting.

Liberal groups, meanwhile, also aided Democratic leadership’s cause. On the heels of MoveOn.org’s March 19 endorsement of the supplemental, the liberal Council for a Livable World backed the bill, saying in a statement that “defeat of the supplemental will hand President Bush the policy and political victory he longs for by enabling the passage of a funding bill with no restrictions or timeline for withdrawal at all.”

The group’s message echoed the one that Democratic leaders delivered to wavering members in private meetings across Capitol Hill on Tuesday.

“There’s no such thing as perfection,” said Rep. Sanford D. Bishop Jr., D-Ga., a war critic and member of the whip team.

Democratic leaders also were trying to stop defections from conservative Blue Dogs worried that the bill sets too many conditions for generals on the battlefield and includes too much non-war spending.

Several Blue Dog Coalition members in the House are opposed to the bill, including Jim Marshall of Georgia. Stephanie Herseth of South Dakota, the group’s whip, predicted that the number of its members voting against the measure would be in the “single digits.”

House GOP members have formed a nearly solid block against the measure, and they are backed by a presidential veto threat.

Some Republicans said they will vote against the measure because of their opposition to the Iraq language, even though their districts would benefit from spending in the bill. Rodney Alexander, R-La., said his state could use the hurricane relief money in the supplemental, but he still couldn’t support the measure. “Yeah, it’s tough to vote against aid for your state, but this is a principle vote,” Alexander said.

Yet several defections were possible from GOP ranks, including Walter B. Jones of North Carolina, and their votes could prove important.

Democratic leaders are leaning toward a closed rule that would not permit amendments on the House floor. And if they do allow them, few would be permitted.

Amendments that may be offered include one by Barbara Lee, D-Calif., that would require that war funding be used only for a withdrawal of U.S. forces from Iraq by year’s end. Another may come from Christopher Shays, R-Conn.; it would require the president to send Congress a timeline for withdrawing U.S. forces and would encourage the Iraqi parliament to vote on the plan. Unless two-thirds of the parliament voted no, it would commence.

The Senate Appropriations Committee’s version of the supplemental is expected to total $121 billion or $122 billion. It was unclear Tuesday whether the panel would include Iraq withdrawal language.

Majority Leader Harry Reid, D-Nev., said he expected the measure to include a provision comparable to a resolution (S J Res 9) that he wrote and that the Senate defeated last week. The measure would require that a withdrawal of most U.S. forces from Iraq begin within four months of enactment and would set a goal of completing the process by March 2008.

But knowledgeable Senate aides said the panel had not resolved the issue. Daniel K. Inouye of Hawaii, the second-ranking Democrat on the committee, said he did not think the full panel would write an Iraq withdrawal provision. “In all likelihood, I do not think it will happen,” he said.

Panel Democrats would have a difficult time approving a measure with such a provision. Democrats have a 15-14 advantage on the committee, but Tim Johnson, D-S.D., is recuperating from a brain hemorrhage, and Ben Nelson, D-Neb., has opposed setting a timeline for withdrawal. The panel’s conservative GOP members are not likely to diverge from the White House’s stance.

Meanwhile, an aide to Senate appropriator Mary L. Landrieu, D-La., said Senate leaders had agreed to include a waiver of a 10 percent local match requirement for Federal Emergency Management Agency disaster assistance. Landrieu is also prepared to offer an amendment for $1.3 billion to bolster levees in Louisiana. The White House is opposed to both provisions.

Susan Ferrechio and Jonathan Allen contributed to this story.