WASHINGTON, March 15 — The Senate on Thursday rejected a Democratic resolution to withdraw most American combat troops from Iraq in 2008, but a similar measure advanced in the House, and Democratic leaders vowed to keep challenging President Bush to change course in Iraq.

The vote in the Senate was 50 against and 48 in favor, 12 short of what was needed to pass, with just a few defections in each party. It came just hours after the House Appropriations Committee, in another vote largely on party lines, approved an emergency spending bill for Iraq and Afghanistan that includes a timeline for withdrawal from Iraq. The House will vote on that legislation next Thursday, setting the stage for another confrontation.

The action in both houses threw into sharp relief the Democratic strategy of ratcheting up the pressure, vote by vote, to try to force the White House to begin withdrawing troops from Iraq. But it also highlighted Republican unity in opposition; in the Senate, only one Republican, Gordon H. Smith of Oregon, voted with the Democrats.

Republican leaders said they counted the day as a victory. “It is clear now that the majority of the Senate opposes a deadline for the withdrawal of troops,” said Senator Mitch McConnell of Kentucky, the Republican leader. Senator Harry Reid of Nevada, the Democratic leader, countered, “The Republicans are rubber-stamping the president’s failed policy. That’s the message here.”

President Bush, speaking at a Republican fund-raising dinner, applauded the senators who voted against a timetable. “Many of those members know what I know: that if American forces were to step back from Baghdad now, before the capital city is more secure, the scale and scope of attacks would increase and intensify,” he said.

The Democratic resolution in the Senate would have redefined the United States mission in Iraq and set a goal of withdrawing American combat troops by March 31, 2008, except for a “limited number” focused on counterterrorism, training and equipping Iraqi forces, and protecting American and allied personnel. The House measure set a withdrawal deadline of Sept. 1, 2008.

The prospects that either the House or the Senate measure would will win final passage were always considered slim, given that the Senate legislation needed a so-called supermajority of 60 to advance. Even so, the White House issued forceful veto threats, sending a clear signal to Republicans where the president stood. The White House also worked behind the scenes this week to keep Republicans on board.

Both parties consider these measures an important political statement, a measure of how far the debate over Iraq has moved in recent months, and a sign of Americans’ discontent with the war.

But Senator Norm Coleman, a moderate Republican from Minnesota who voted against the Democratic measure, argued that the final vote could still be misleading. “There is frustration and deep concern about the war,” said Mr. Coleman, who is facing a tough re-election fight next year.

As they left the Senate floor, several other moderate Republicans who are facing difficult re-election campaigns next year were quick to register their opposition to the president’s overall Iraq strategy. But they said they were leery of legislating a troop pullout to begin within four months.

“That is such a short time frame for withdrawal,” said Senator Susan Collins, a Maine Republican, who opposed the president’s plan to send more troops to Iraq.

In the end, the Senate resolution did not attract the contingent of seven Republican moderates who joined Democrats in opposing Mr. Bush’s troop buildup plan last month. The only Republican defection was Mr. Smith of Oregon, who said in a statement, “Setting specific dates for withdrawal is unwise, but what is worse is remaining mired in the quicksand of the Sunni-Shia civil war.”

Two Democratic Senators, Ben Nelson of Nebraska and Mark Pryor of Arkansas, crossed party lines to oppose the withdrawal plan. Senator Joseph I. Lieberman, an independent and staunch supporter of Mr. Bush’s Iraq policy, voted as expected with the Republicans. Senator John McCain, an Arizona Republican running for president, was campaigning in Iowa at the time of the vote.

Democrats asserted that the only alternative to their plan was endorsing, once again, the status quo in Iraq. In a debate steeped in anger and dismay, Senator Robert C. Byrd of West Virginia declared, “We were wrong to invade, we were wrong to think victory would be quick or easy, and we are wrong to stay on in occupation that earns us only hatred — with no end, no end, no end in sight.”

Republicans declared that the resolution would be devastating to the American war effort, “like sending a memo to our enemy,” or “giving notice to the other side of when we’re going to depart,” in the words of Mr. McConnell.

The Senate also voted overwhelmingly on Thursday in favor of a pair of nonbinding resolutions, one Democratic and one Republican, expressing support for the troops in Iraq and pledging to provide them with all necessary funds. Republicans have asserted that Democratic policies to end the war will eventually lead to a financing cut that will harm the troops. Democrats furiously deny that charge and have seized on the scandal over poor conditions at the Walter Reed Army Medical Center as evidence that Republicans are not true champions of the troops.

Despite the flurry of votes, the Iraq debate in the Senate is far from over. Senate Democrats said they would try to influence the president’s Iraq policy when they begin taking up the administration’s military spending request next week.

Across the Capitol, the House Appropriations Committee advanced its version of that legislation by a vote of 36 to 28. It was considered a major test vote, with Representative Barbara Lee of California the lone Democrat voting against it. “The American people sent a mandate to us to bring home our men and women before the end of the year,” Ms. Lee said. “I don’t think the president deserves another chance.”

As she spoke, two protesters sat in the back of the hearing room, holding a sign handwritten with black ink on pink paper that said: “Wake up. Stop Buying Bush’s War.” Other antiwar activists milled about outside the committee room, occasionally confronting lawmakers as they came and went.

Largely because of the strength of antiwar sentiment in the House Democratic caucus, and complaints that the legislation’s timetable is not fast enough, party leaders still face a fragile majority when they bring this legislation to the full House next week. While the House proposal calls for most American combat troops to be removed from Iraq no later than Aug. 31, 2008, it would require the drawdown to start up to a year earlier if the Iraqi government cannot show progress.

The plan also places conditions on the war financing, including a requirement that troops receive proper training, equipment and a period of rest between deployments. As a gesture to conservatives, the legislation would allow the president to waive those requirements on national security grounds.

“In World War II, troops were in action 30, 40, 50 days and then got relief,” said Representative John P. Murtha, a Pennsylvania Democrat. “Now, we don’t have the troops to relieve them.”

But Representative Harold Rogers, a Kentucky Republican, accused Democrats of loading up the legislation — which now has a price tag of $124 billion — with an array of sweeteners, simply to draw support for a controversial plan to bring closure to the Iraq war.

“Welcome Kmart shoppers,” Mr. Rogers said. “This is the shopping mart for those who are nervous about supporting the precipitous withdrawal of troops. This is an effort to buy votes.”