"The moral thing to do is not to stand by and do nothing," he declared at a news conference in Stockholm with Swedish Prime Minister Fredrik Reinfeldt.
With Obama in Europe, the president's top national security aides were briefing legislators in a series of public and private hearings, hoping to advance their case for limited strikes against Syrian President Bashar Assad's regime in retaliation for what the administration says was a deadly sarin gas attack by his forces outside Damascus last month.
The Senate Foreign Relations Committee's vote would be the first in a series as the president's request makes its way through Senate and House committees before coming before the two chambers for a final vote. But with some senators saying the resolution is too strong and others believing it too weak, Sen. John Barrasso, R-Wyo., said a vote could be delayed.
After briefing the committee in private, Secretary of State John Kerry was asked whether it was too soon for a vote, and said: "You have to ask the gentlemen. We had a good meeting."
In an initial survey, the AP found 17 senators supporting or leaning in favor of the resolution approving a U.S. military response in Syria, and 14 against or leaning against it. There were 69 senators who either said they were undecided or whose views were unknown. Of those supporting or leaning in favor of the resolution, 13 were Democrats and four were Republicans. Those against or leaning against the resolution were 2 Democrats, 11 Republicans and one independent.
Sending a message to Congress from afar, Obama insisted there was far more than his own credibility at stake.
"I didn't set a red line, the world set a red line," he said. "The world set a red line when governments representing 98 percent of world population said the use of chemical weapons are abhorrent." He added that "Congress set a red line when it ratified that treaty."