For much of last week, it appeared Obama was ready to launch a strike against Assad's government without authorization from either the U.N. or the U.S. Congress. But he made an abrupt reversal on Saturday, announcing he would hold off on a military response and ask Congress for a vote in support. The White House has refused to say whether Obama would go forward with a strike if lawmakers vote against using force.
With one eye on Washington, Obama on Thursday lobbied lawmakers from afar, and he canceled a planned trip to California for next week to stay in Washington and make his case as votes near. Back home, his administration continued its full-scale sales job with another round of closed-door meetings for lawmakers about intelligence on Syria.
Sen. Dianne Feinstein, D-Calif., chairwoman of the Senate Intelligence Committee, said officials showed a DVD on chemical weapons with "what pinpointed eyes mean, what the convulsions mean" when nerve agents affect people. She said all senators would receive a copy.
"It's horrendous," Feinstein said.
The administration has focused on influencing lawmakers who will vote, but opinion polls show little desire by the general public for military intervention in Syria.
"It weighs on me," Feinstein said of the lack of popular support. "There's no question: What's coming in is overwhelmingly negative."
An AP survey of senators found 34 supporting or leaning toward military action, 26 opposed or leaning against, and 40 undecided.
Among the undecided senators is Ron Wyden, a Democrat from Oregon.
"What the effects of a military strike would be are not clear," Wyden said.
While intelligence information on the recent chemical weapons attack has been provided to members of Congress, the public hasn't had the same access - a point Feinstein recognized, saying: "But you see, then they don't know what I know. They haven't heard what I've heard."
The Senate resolution authorizes the "limited and specified use" of the U.S. armed forces against Syria, restricts military action to 90 days and bars American ground troops from combat.
The measure is expected to reach the Senate floor next week. The timetable is more uncertain in the Republican-led House, where the resolution is expected to face greater opposition. House Speaker John Boehner, R-Ohio, and House Majority Leader Eric Cantor, R-Va., have both backed military action in Syria, but there's no guarantee they'll be able to sway enough other members of their party.
Despite widespread condemnation of the chemical weapons attack from allies, there are few countries likely to join the U.S. in undertaking military action if Obama moves forward with a strike. France has been the most favorable, though French officials said they were awaiting the outcome of the votes in Washington and would not proceed alone.
Obama and French President Francois Hollande were to hold a meeting on the sidelines of the summit Friday.