The 28-70 vote was 32 short of the 60 needed to cut off a GOP filibuster. The legislation, sponsored by Senate Majority Leader Harry Reid and Sen. Russ Feingold, was indicative of the Democratic leadership's new hardline strategy.
Unable to attract enough Republican support on less contentious proposals, Reid has sought votes on strong anti-war measures intended to force a withdrawal of troops.
Also Thursday,to condemn an advertisement by the liberal anti-war group MoveOn.org that accused the top military commander in Iraq of betrayal.
The full-page ad that appeared in The New York Times last week was headlined: "General Petraeus or General Betray Us? Cooking the books for the White House."
Senate Democrats' efforts to challenge President Bush's Iraq policies were dealt a demoralizing blow Wednesday after they failed to scrape together enough support to guarantee troops more time at home.
The bill was the weakest of the Democratic proposals and the one thought to have the most appeal to republicans, reports CBS News correspondent Bob Fuss. But after Sen. John Warner, R-Va., switched positions from supporting to opposing it, it's fate was sealed.
The 56-44 vote - four short of reaching the 60 needed to advance - all but assured that Democrats would be unable to muster the support needed to pass tough anti-war legislation by year's end. The legislation, sponsored by Sen. Jim Webb, D-Va., was seen as the Democrats' best shot because of its pro-military premise.
"The idea of winning the war in Iraq is beginning to get a second look," said Sen. Lindsey Graham, R-S.C., who led opposition to the bill alongside Sen. John McCain.
Webb's legislation would have required that troops spend as much time at home training with their units as they spend deployed in Iraq or Afghanistan. Members of the National Guard or Reserve would be guaranteed three years at home before being sent back.
Most Army soldiers now spend about 15 months in combat with 12 months home.
"In blocking this bipartisan bill, Republicans have once again demonstrated that they are more committed to protecting the president than protecting our troops," said Reid.
Wednesday's vote was the second time in as many months that Webb's bill was sidetracked. In July, a similar measure also fell four votes short of advancing.