The U.S., citing intelligence reports, says sarin gas was used in the Aug. 21 attack outside Damascus, and that 1,429 people died, including 426 children.
The number is higher than that, said Khalid Saleh, head of the press office at the anti-Assad Syrian Coalition who was in Washington to lobby lawmakers to back Obama. Some of those involved in the attacks later died in their homes and opposition leaders were weighing releasing a full list of names of the dead.
But the Britain-based Syrian Observatory for Human Rights, which collects information from a network of anti-government activists, says it has so far only been able to confirm 502 dead.
In an interview Sunday, Assad told U.S. journalist Charlie Rose there is not conclusive evidence about who is to blame and again suggested the rebels were responsible. From Beirut, Rose described his interview that is set to be released Monday on television shows hosted by Rose.
At the same time, Obama has planned his own public relations effort. He has scheduled six network interviews on Monday and then a nationally televised speech from the White House on Tuesday, the eve of the first votes in Congress.
On Wednesday, the Democratic-led Senate is expected to hold the first showdown vote over a resolution that would authorize the "limited and specified use" of U.S. armed forces against Syria for no more than 90 days and barring American ground troops from combat. A final vote is expected at week's end.
A vote in the Republican-controlled House of Representatives appears likely during the week of Sept. 16.
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, a California Democrat: "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."
Nearly half of the 433-member House of Representatives 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, a conservative Texas Republican.
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, the spokesman for the Western-backed opposition seeking U.S. strikes.
On Saturday, a U.S. official released a DVD compilation of videos showing attack victims that the official said were shown to senators during a classified briefing on Thursday. The images have become a rallying point for the administration.
"Those videos make it clear to people that these are real human beings, real children, parents being affected in ways that are unacceptable to anybody, anywhere by any standards," Secretary of State John Kerry said in Paris. "And the United States of America that has always stood with others to say we will not allow this this is not our values, it's not who we are."
But McDonough conceded the United States doesn't have concrete evidence Assad was behind the chemical attacks.
Recent opinion surveys show intense American skepticism about military intervention in Syria, even among those who believe Syria's government used chemical weapons on its people.
Congress, perhaps, is even more dubious.
"It's an uphill slog," said Rep. Mike Rogers, the Republican chairman of the House Intelligence Committee who supports strikes on Assad.
"I think it's very clear he's lost support in the last week," Rogers added, speaking of the president.
Complicating the effort in the Senate is the possibility that 60 votes may be required to authorize a strike. Republican Sen. Rand Paul of Kentucky said he would consider using Senate procedural maneuvers to delay shutting off debate, but noted such a tactic was unlikely to permanently block a vote. Paul would, however, insist his colleagues consider an amendment to the resolution that would bar Obama from launching strikes if Congress votes against the measure.
Still, Sen. Harry Reid, the Democratic leader, has predicted authorization of military strike against Syria.
While the publicly discussed information lacks a direct link between Assad and the use of chemical weapons, the private briefs are no better, two lawmakers said.
"The evidence is not as strong as the public statements that the president and the administration have been making," said Republican Rep. Justin Amash of Michigan. "There are some things that are being embellished in the public statements. ... The briefings have actually made me more skeptical about the situation."
Republican Rep. Buck McKeon of California, said "they have evidence showing the regime has probably the responsibility for the attacks."
But that's not enough to start military strikes. "They haven't linked it directly to Assad, in my estimation," said McKeon, chairman of the House Armed Services Committee.
McDonough, an Obama foreign policy adviser dating back to his 2008 presidential campaign, said the dots connect themselves.
"The materiel was used in the eastern suburbs of Damascus that have been controlled by the opposition for some time," McDonough said. "It was delivered by rockets rockets which we know the Assad regime has and we have no indication that the opposition has."
At the same time, McDonough acknowledged the risks that military action could drag the U.S. into the middle of a brutal civil war and endanger allies such as Israel with a retaliatory attack.
The U.S. is "planning for every contingency in that regard and we'll be ready for that."
Congress resumes work Monday after its summer break, but already a heated debate is under way about Syria.
Vice President Joe Biden planned to host a dinner Sunday night for a group of Senate Republicans.
Obama's national security adviser, Susan Rice, plans to discuss Syria in a speech Monday at the New America Foundation and later meet with members of the Congressional Black Caucus. A bipartisan, classified briefing for Congress is set for Monday and another is slated for Wednesday.
McDonough spoke with ABC's "This Week," CBS' "Face the Nation," NBC's "Meet the Press," CNN's "State of the Union" and "Fox News Sunday." McCaul and Sanchez were on NBC. Cruz appeared on ABC. Rogers and Amash spoke to CBS. Paul was interviewed on Fox. McKeon was on CNN.
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Associated Press writers Philip Elliott in Washington and Deb Reichmann and Sarah DiLorenzo in Paris contributed to this report.
