He said that if the United States does not stand up to Assad and against the use of chemical weapons, some world figures will believe "it's OK to use them with impunity."
The argument failed to sway two Republican senators, who announced on Thursday that they would vote against any military action. Sens. David Vitter of Louisiana and Mike Lee of Utah, members of the Armed Services Committee, participated in briefings on Wednesday with Defense Secretary Chuck Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff.
Lee said in a statement that the risks of the president's plan outweigh the gains, while Vitter said he was concerned "that getting involved in Syria, after Iraq and Afghanistan, would make mustering our resolve to stop a nuclear Iran impossible."
Days from a vote, an Associated Press survey of senators found 34 supporting or leaning toward military action, 26 opposed or leaning against and 40 undecided.
Secretary of State John Kerry, testifying for the second consecutive day before Congress, insisted that the U.S. military response would be restricted as Americans fatigued by more than a decade of war in Iraq and Afghanistan show little inclination to get involved in Syria.
"I don't believe we're going to war, I just don't believe that," Kerry told the House Foreign Affairs Committee, citing the ground troops and long-term commitment that he said wars entail. "That's not what we're doing here. The president is asking for permission to take a limited military action, yes, but one that does not put Americans in the middle of the battle."
In the Senate, five Republicans, including potential presidential candidates Rand Paul and Marco Rubio, and two Democrats opposed the resolution, which is expected to reach the Senate floor next week. The timing of a vote is uncertain.