WASHINGTON - Believing they have been given a clear mandate from voters, Democrats are trying to challenge President Bush on the Iraq war while struggling to find enough votes to do it.

Party leaders are facing a caucus deeply divided on the issue and hold only a narrow majority in Congress. With their hands tied if just a few members stray, Democratic leaders are finding it tough to pass legislation that would require Bush to start bringing troops home.

House debate on an anti-war measure was expected to begin Thursday, with a vote the following day, while a Senate committee planned to vote Thursday on a similar measure.

The stakes are high for Congress' new Democratic leadership, which wants to prove it can govern, influence Bush's war policy and still support the military.

“If they fail to provide our troops with what they need it's on their backs,” said Republican Rep. Eric Cantor of Virginia.

The House's $124 billion spending bill would fund the wars in Iraq and Afghanistan and require that combat troops leave Iraq by fall of 2008, and possibly sooner if the Iraqi government does not make progress on its political and security commitments.

But several hurdles remain. Several anti-war liberals are expected to join Republicans in opposing the measure because they say it continues to bankroll an immoral war. And if the bill does scrape by in the House, it may sink in the Senate, where many Democrats have resisted firm timetables on the war.

On top of that, President Bush has vowed to veto such a restrictive measure if it ever reaches his desk.

House Speaker Nancy Pelosi, D-Calif., continued Wednesday to press party members to back the bill, unsure whether she had enough votes to pass it. In a closed-door meeting, former President Carter's national security adviser, Zbigniew Brzezinski, tried to convince party skeptics that the bill was their best chance at ending the war.

Pelosi initially had planned for a final vote Thursday but pushed it off until Friday, a tactic that gives her more time to ensure she has the 218 votes to pass it.

“This is not going to go anywhere,” said Rep. Lynn Woolsey, D-Calif., who wants legislation to end the war immediately. “So if you're going to be symbolic, be bold.”

But some of Woolsey's colleagues say it's not that easy. Rep. Elijah Cummings, D-Md., former chairman of the Congressional Black Caucus, said he feels the heat from voters who do not want another penny to go toward the war.

“But I'm thinking about if the bill fails, what happens?” Cummings said. “If the bill fails, we start from scratch.”

Democrats then would be forced to pass spending legislation without the deadlines, whereas the current bill would at least send Bush a message that Congress is not behind the war, he said. “I think when all the dust settles, no matter what, we're going to have troops in Iraq. And so long as they're there, I have a duty to protect them and provide them what they need,” Cummings said.

In the Senate, the Appropriations Committee bill would require troops to start coming home in four months. Unlike the House bill, which sets a firm deadline for combat operations to cease, the $122 billion Senate bill identifies a nonbinding goal of getting troops out by March 31, 2008.

Both measures would allow an unspecified number of troops to be left behind to conduct anti-terror missions, train Iraqi forces and protect diplomatic personnel. Of the more than 140,000 U.S. troops in Iraq, fewer than half are combat forces.

The Senate bill is similar to a resolution rejected last week. It failed on a 50-48 procedural vote, falling 12 votes shy of the 60 needed to move forward to final vote.

But Democrats think the spending legislation has a better chance of passing. Sen. Ben Nelson, who voted against last week's resolution, has agreed to support the spending legislation because of language added outlining benchmarks for the Iraqi government.

Republican leaders and the White House say they will reject the bill.

“It is unfortunate that the Senate is wanting to delay vital funds for our troops by producing a bill that mirrors House legislation that will never become law, attempts to tie the hands of our military commanders and is a Christmas wish list of non-war related spending add-ons,” said Sean Kevelighan, spokesman for the White House budget office.