Setting the stage for an intense week of lobbying in Washington over the strike resolution, Obama said he planned to make his case to the American people in an address Tuesday night.
"It's conceivable at the end of the day I don't persuade a majority of the American people that it's the right thing to do," Obama acknowledged. "And then each member of Congress is going to have to decide."
Obama, speaking at the end of a two-day Group of 20 economic summit, earlier held a surprise meeting with Russian President Vladimir Putin, a chief opponent of U.S. military action. Both Obama and Putin said that while they still disagreed, the meeting was constructive.
Obama, in his post-summit news conference, seemed to be feeling the burden of the challenge he faces in persuading the American public, the international community and Congress to back military action. But he expressed confidence the American people and lawmakers, weary after long-running wars in Iraq and Afghanistan, would listen.
"I trust my constituents want me to offer my best judgment. That's why they elected me. That's why they re-elected me," he said.
He said he couldn't honestly claim there was an imminent threat to the United States from the chemical weapons use in Syria. But he argued action was essential to uphold prohibitions against the use of weapons of mass destruction.
Ten members of the Group of 20 joined the United States in a joint statement accusing the Syrian government of carrying out a chemical weapons attack on civilians last month and calling for a strong international response against the government of Syrian President Bashar Assad. The statement stopped short of explicitly calling for military action against Syria.
The countries signing the statement with the U.S. were Australia, Canada, France, Italy, Japan, South Korea, Saudi Arabia, Spain, Turkey and the United Kingdom.
Seeking to rally support back in Washington, the administration planned another classified briefing for all lawmakers next Monday night after Congress returns, and White House Chief of Staff Denis McDonough planned to attend the closed-door Democratic caucus meeting Tuesday morning, according to a congressional aide.
Although surveys showed a significant number of House Republicans and Democrats opposed to military action or leaning against it, officials in the leadership insisted it was premature to say the resolution could not be approved. At this stage, just a third of the House and Senate have participated in classified briefings and Obama is still reaching out to lawmakers.
Still, final passage rests on significant votes from House Republicans and Democrats, and the administration is struggling to reach those numbers.
Obama said he and other leaders at the summit had had a "full airing of views on the issue" during a three-hour dinner Thursday night. He said many foreign nations would be issuing statements on their positions, but he didn't say whether any specifically had joined France in supporting his move toward U.S. military strikes.
He said the leaders were unanimous in believing that chemical weapons were used in Syria and that international norms against that use must be maintained. He said division comes over how to proceed through the United Nations.
Obama also held an unannounced meeting with Russian President Putin, a staunch ally of Assad.
Putin called the discussion "substantial and constructive." Likewise, Obama said it was a "candid and constructive conversation." The U.S. president said they agreed the underlying conflict in Syria could only be resolved through a political transition. Obama also said he thinks it is important that he and Putin work together to urge all sides in the conflict to try to resolve it.
Putin said they didn't discuss the case of National Security Agency leaker Edward Snowden, who is living in Russia on temporary asylum from prosecution in the United States for leaking classified programs. Instead, he said the entire meeting focused on airing their positions on Syria.
"I don't agree with his arguments and he doesn't agree with mine, but we are listening to them and trying to analyze them," Putin said.
Russia on Friday warned the United States and its allies against striking any chemical weapon storage facilities in Syria. The Russian foreign ministry said such targeting could release toxic chemicals and give militants or terrorist access to chemical weapons.
"This is a step toward proliferation of chemical weapons not only across the Syrian territory but beyond its borders," the Russian statement said.
Meanwhile, the Kremlin said Russia was boosting its naval presence in the Mediterranean Sea, moving in warships into the area and stoking fears about a larger international conflict if the United States orders airstrikes.
Moreover, China remained a firm no. The European Union is skeptical about whether any military action can be effective. Even Pope Francis weighed in, urging leaders gathered here to abandon what he called a "futile mission."
Still, Obama was undeterred. He and French President Francois Hollande, the U.S.'s strongest ally on Syria and a vocal advocate for a military intervention, met on the sidelines of the summit about attracting European support for action. "It's clear that there are many countries that agree with us that international norms must be upheld," Obama said.
Said Hollande: "To do nothing would mean impunity."
British Prime Minister David Cameron said his country will not be part of military action because of opposition from Parliament. He said the international opinion is also divided.
"This summit was never going to reach agreement," he said, but added the case made by Obama and other countries "was extremely powerful."
Illustrating the risks associated with a strike, the State Department ordered nonessential U.S. diplomats to leave Lebanon, a step under consideration since last week when Obama said he was contemplating military action against the Syrian government.
Before his scheduled return to Washington late Friday, Obama also planned to meet with Russian lesbian, gay, bisexual and transgender activists, calling attention to another area of disagreement with Moscow.
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