The survey shows that roughly six in 10 Americans want their representatives in Congress to vote "no" on giving the president the authority to use force. Though Obama argued that the Constitution gives him the authority to strike even if Congress is against it, a similar majority oppose him doing so. Only about a third back military action.
Support rises slightly when the proposed military action is described as missiles launched from Navy ships. On that question, 44 percent express support, while 51 percent are opposed.
Behind the public's hesitation is uncertainty over whether military action would be in America's national interest. Just 24 percent say yes, 47 percent say no, and 28 percent lack enough information to have an opinion. By 54 percent to 33 percent, Americans say Obama has yet to make a convincing case.
Obama approaches his 9 p.m. EDT speech with a tepid overall approval rating of 45 percent, while 50 percent disapprove of his job performance. On Syria specifically, 28 percent approve his handling of the situation, while 57 percent disapprove.
Skepticism about an attack partly reflects the nation's fatigue after 12 years of war in Iraq and Afghanistan since the 9/11 attacks. The poll shows Americans are ambivalent about how successful those wars have been.
Just 39 percent say the nation is safer than before 9/11. Another 33 percent say the nation is about as safe as before 9/11, while 28 percent say it is less safe.
The telephone survey of 1,000 adults, conducted Sept. 5 to Sept. 8, carries a margin of error of 3.1 percentage points.
—By CNBC's John Harwood. Follow him on Twitter @JohnJHarwood.