Democratic senators had tabled two proposals on Thursday to begin withdrawing most of the 127,000 American soldiers in Iraq.
Republicans backed George Bush's policies on Iraq to defeat the plans and accused Democrats of "cutting and running".
Under the first proposal, offered by Democratic Senator John Kerry and supported by 12 other Democrats but no Republicans, soldiers would have started to leave Iraq by the end of the year. A deadline of July 2007 would have been set for all combat forces to pull out.
"Redeploying United States troops is necessary," said Kerry.
Most senators did not agree, and the proposal fell on an 86-13 vote.
Minutes later, a resolution to urge the administration to begin "a phased redeployment of US forces" sometime this year was defeated by 60 votes to 39. The resolution would not have set a deadline for the end of the US presence in Iraq.
Bill Frist, the Republican Senate Majority Leader, said: "The Senate defeated a bad policy that threatens our national security and poses unacceptable risks to Americans."
Bush has repeatedly said that US soldiers will stay in Iraq until their security forces can defend the country and deal with the uprising that began after the 2003 US-led invasion defeated Saddam Hussein.
Gen Casey expects US forces in
Iraq to be scaled down
Dick Cheney, the US vice president, had criticised the Democrats, saying: "Absolutely the worst possible thing we could do at this point would be to validate and encourage the terrorists by doing exactly what they want us to do, which is to leave."
But Senate Democratic leader Harry Reid of Nevada said: "Demanding a change of course is not irresponsible. It's not unpatriotic. It is the right thing to do."
Democrats suggested that the Republicans would pay for blindly following Bush's policies when legislative elections are held in November. Democrats are seeking to capitalise on voters' discontent over the Iraq war.
Despite the defeat for the withdrawal plans, the top US commander in Iraq said he expected a reduction in forces this year, adding that he did not agree with congressional efforts to put a timetable on the effort.
"I don't like it, I feel it would limit my flexibility" and give the enemy a schedule to focus on, General George Casey said a Pentagon briefing.
Casey said the reductions would be gradual as Iraqi security forces take responsibility for more territory.