The Senate panel's vote marked the first formal response in Congress, four days after Obama unexpectedly put off an anticipated cruise missile strike against Syria and instead asked lawmakers to unite behind such a plan.
Obama was in Sweden after a day of diplomacy when the vote occurred. White House press secretary Jay Carney praised the Senate committee for backing Obama's call for a strike, saying the measure would uphold U.S. national security interests.
At a news conference earlier, Obama said, "I always preserve the right and responsibility to act on behalf of America's national security." In a challenge to lawmakers back home, he said Congress' credibility was on the line, not his own, despite saying a year ago that the use of chemical weapons would cross a "red line."
In Washington, Secretary of State John Kerry and other top administration officials made the case for action at the first House hearing on Obama's request for congressional backing.
Kerry said Assad had used chemical weapons 11 times, including once last spring. At that time, he said, Obama did not have a "compelling" enough case to push for a U.S. military response.
As for the most recent chemical weapons attack, Kerry declared that "only the most willful desire to avoid reality can assert that this did not occur as described or that the regime did not do it. It did happen - and the Assad regime did do it."
Asked about international support for Obama's threatened military strike, Kerry said the Arab League has offered to pay the cost of any U.S. military action. He was not specific but said the offers have been "quite significant, very significant."
Few if any members of Congress dispute the Obama administration's claim that Assad was responsible for the attack, and lawmakers in both parties appear far more focused on determining how they should respond.
The Senate committee's vote capped a hectic few days in which lawmakers first narrowed the scope of Obama's request limiting it to 90 days and banning combat operations on the ground and then widened it.
Republican Sen. John McCain, a proponent of aggressive U.S. military action in Syria, joined forces with Democratic Sen. Chris Coons to add a provision calling for "decisive changes to the present military balance of power on the ground in Syria."
At their urging, the measure was also changed to state that the policy of the United States is "to change the momentum on the battlefield in Syria so as to create favorable conditions for a negotiated settlement that ends the conflict and leads to a democratic government in Syria." McCain, who has long accused Obama of timidity in Syria, argued that Assad will be willing to participate in diplomatic negotiations only if he believes he is going to lose the civil war he has been fighting for over two years.
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.
Republican Rep. Ed Royce said that while it would be important to deter the use of chemical weapons by Assad and others, there remained many unanswered questions, including what the U.S. would do if Assad retaliated.
"The administration's Syria policy doesn't build confidence," he said.
In a letter to her rank and file, Democratic Leader Nancy Pelosi said she had received suggestions for legislation in the House "to add language to prevent boots on the ground, to tie the authorization more closely to the use of chemical weapons and to address concerns about an open-ended timetable."
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Associated Press writers David Espo, Bradley Klapper, David Espo, Julie Pace, Josh Lederman, Donna Cassata, Alan Fram, Jennifer C. Kerr and Lolita C. Baldor contributed to this report.
