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 today, with a vote the following day, while a Senate committee planned to vote today 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 Rep. Eric Cantor, R-Virginia.