Republican Senator John McCain, an outspoken advocate of intervention against Assad's regime, said today that he doesn't support the Senate resolution.
He expressed support for the administration's plan after meeting with Obama at the White House on Monday, but he has wanted more support to Syria's rebels.
The White House had no immediate reaction to the Senate measure. Secretary of State John Kerry, testifying earlier before the committee, signaled that the troop restriction was acceptable.
Now the administration needs to persuade a Republican-dominated House of Representatives has opposed almost everything on Obama's agenda since the party seized the majority more than three years ago.
The top opposition Republican in Congress, House Speaker John Boehner, has signaled key support, saying the US has "enemies around the world that need to understand that we're not going to tolerate this type of behavior."
The administration says 1,429 people died from the gas attack on Aug. 21. The British-based Syrian Observatory for Human Rights, which collects information from a network of anti-government activists in Syria, says its toll has reached 502. Assad's government blames the episode on the rebels. A United Nations inspection team is awaiting lab results on tissue and soil samples it collected while in the country last week.
Obama on Saturday unexpectedly stepped back from ordering a military strike under his own authority and announced he would seek congressional approval.
On Wednesday, Kerry, Defense Secretary Chuck Hagel and the chairman of the Joint Chiefs of Staff, Gen. Martin Dempsey, were trying to make their case in a public hearing before the House Foreign Affairs Committee. They and other senior administration officials also were providing classified briefings to the Senate Foreign Relations and Armed Services committees.
But even supporters of military action urged Obama to do more to sell his plans to an American public that is highly skeptical after a decade of war in Iraq and Afghanistan.
Obama is expected to find little international support for action. Among major allies, only France has offered publicly to join the United States in a strike. The United Nations secretary-general has warned that any "punitive" strike on Syria would be illegal without a sound case for self-defense or the approval of the Security Council, where Syria ally Russia has used its veto power to block action against Assad's regime.
Russian President Vladimir Putin has warned the West against taking one-sided action in Syria but said Russia "doesn't exclude" supporting a UN resolution on punitive military strikes if it is proved that Syria used poison gas on its own people.