The White House is making a big push to rally members of Congress and the American public behind U.S. President Barack Obama's plan for a U.S. military strike against Syria.
His administration says the government of Syrian President Bassar al-Assad used chemical weapons in an attack last month near Damascus, and that a strong U.S. response is needed to deter the future use of deadly chemicals.
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- Read about the limited options U.S. has for a military strike
Syria claims rebels carried out the Aug. 21 attack.
Obama's chief of staff, Denis McDonough, is set to make the White House's case during interviews on five Sunday talk shows.
Vice President Joe Biden plans to host a dinner Sunday night for a group of Senate Republicans. And lawmakers should expect more phone calls from top officials.
Obama is giving a national address Tuesday night.
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 resumes work Monday after its summer break, but already a heated debate is underway about Syria.
On Wednesday, the first showdown Senate vote is likely over a resolution authorizing 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 House vote appears likely during the week of Sept. 16.
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. The Senate is more evenly divided ahead of its vote.
Nearly half of the 433-member House and a third of the 100-member Senate remain undecided.
Complicating the effort in the Senate is the possibility that a three-fifths majority may be required. Republican Sen. Rand Paul of Kentucky says he is going to filibuster.
Still, Sen. Harry Reid, the Democratic leader, has predicted, "I think we're going to get 60 votes,"
Another bipartisan, classified briefing for Congress is set for Monday. McDonough plans to meet Tuesday with the House Democratic Caucus, whose support could be crucial as Obama faces opposition from House Republicans.
Obama's national security adviser, Susan Rice, plans to discuss Syria in a speech Monday at the New America Foundation.
Graphic new video of victims
At least 150 people picketed outside the White House on Saturday to oppose any military action against Syria. Demonstrations also took place in New York City, Boston and Indianapolis and in Louisiana and Michigan.
Also Saturday, a U.S. official released a DVD compilation of videos showing victims of the Aug. 21 attack. The DVD was shown to senators during a classified briefing on Thursday, and some of the videos were first broadcast on CNN. Supporters of the Syrian rebels had posted the videos on YouTube.
The videos have also been posted on the U.S. Senate Select Committee on Intelligence's website.
The humanitarian group Doctors Without Borders estimated on Aug. 24 that 355 people were killed in the chemical attack.
The Obama administration says 1,429 people died in 12 locations mostly east of the capital.
The Britain-based Syrian Observatory for Human Rights says its current total stands at 502.
John Kerry seeks support in Europe
Meanwhile, U.S. Secretary of State John Kerry is continuing his European tour with his goal of trying to gain support for military action in Syria.
After meeting Arab League foreign ministers in Paris on Sunday, Kerry said they agreed the end of Syria's civil war will require a political solution.
He also and stressed the U.S. is not seeking long-term involvement in the conflict.
"There is no military solution," Kerry said. "And what the U.S. is seeking, together with others, not alone, but with others in increasing numbers, is to enforce the standard with respect to the use of chemical weapons. We are not seeking to become engaged in, or party to, or take over Syria's civil war.
"But as we discussed today, all of us agreed, not one dissenter, that al-Assad's deplorable use of chemical weapons, which we know killed hundreds of innocent people, including at least 426 children, on this occasion, this crosses an international, global red line," Kerry said.
With files from CBC News