Asked about Assad's claims there is no evidence he used the weapons, Secretary of State John Kerry told reporters in London: "The evidence speaks for itself."
At the same time, Obama has planned his own public relations effort. He has scheduled six network interviews on Monday and then a primetime speech to the nation from the White House on Tuesday, the eve of the first votes in Congress.
Obama faces a tough audience on Capitol Hill. A survey by The Associated Press shows that House members who are staking out positions are either opposed to or leaning against Obama's plan for a military strike by more than a 6-1 margin.
"Lobbing a few Tomahawk missiles will not restore our credibility overseas," said Rep. Mike McCaul, the Texas Republican who chairs the House Homeland Security Committee.
Added Rep. Loretta Sanchez, D-Calif.: "For the president to say that this is just a very quick thing and we're out of there, that's how long wars start."
Almost half of the 433-member House and a third of the 100-member Senate remain undecided, the AP survey found.
"Just because Assad is a murderous tyrant doesn't mean his opponents are any better," said Sen. Ted Cruz, R-Texas.
But some of Assad's opponents are pleading for aid.
"The world is watching, and Syrians are wondering: When is the international community going to act and intervene to protect them?" said Saleh.
On Saturday, a U.S. official released a DVD compilation of videos showing attack victims that the official said were shown to senators during Thursday's classified briefing. The graphic images have become a rallying point for the administration. Sen. Dianne Feinstein, D-Calif., chairwoman of the Intelligence Committee, also posted videos on the committee's website.
But McDonough conceded the United States doesn't have concrete evidence Assad was behind the chemical attacks.