STRELNA, Russia -- President Barack Obama raced home Friday to confront one of the biggest tests of his presidency, as he ramped up a campaign to persuade Congress to support air strikes against Syria that many world leaders he had consulted declined to back.
After two days of lobbying that included a vigorous dinner debate that went into the early-morning hours, Mr. Obama failed to forge an international consensus behind military action, as other leaders urged him not to attack without United Nations' permission. But he won agreement from some allies blaming Syria's government for a chemical weapons attack and endorsing unspecified actions.
The deep divisions on display at the Group of 20 summit meeting raised the stakes even further for Mr. Obama as he seeks authorization from Congress for a "limited, proportionate" attack. He hoped to use the statement from allies condemning Syria to leverage more domestic support, but he acknowledged that he had a "hard sell" and might fail to win over an American public that polls show still opposes a strike.
Mr. Obama ordered aides to fan out in coming days with a series of speeches, briefings, phone calls and TV appearances to sway both Democrats and Republicans reluctant to get involved in yet another Middle East war. He also announced that he would address the nation Tuesday evening from the White House to lay out his case before Congress votes.
"Failing to respond to this breach of this international norm would send a signal to rogue nations, authoritarian regimes and terrorist organizations that they can use WMD and not pay a consequence," he said at a news conference, using the acronym for weapons of mass destruction. "And that's not a world we want to live in."
The return to the Washington fray came after a tense overseas trip punctuated by an extraordinary showdown with the meeting's host, Russian President Vladimir Putin, who not only opposes a strike, but also dismisses the notion that Syria's government gassed its own people.
During a long, late-night discussion about Syria, the two presidents effectively competed for the support of the other leaders, each man arguing his position and soliciting peers as if they were voters. At the end, Mr. Putin said a majority of the leaders joined him in opposing a military strike independent of the United Nations, including the leaders of China, India, Indonesia, Argentina, Brazil, Italy, Germany and South Africa.
"We hear each other and understand the arguments," Mr. Putin said. "We simply don't agree with them. I don't agree with his arguments, and he doesn't agree with mine, but we hear and try to analyze."
The only members of the Group of 20 nations that supported Mr. Obama's plan, the Russian leader said, were Canada, France, Saudi Arabia and Turkey, all that were on Mr. Obama's side when he arrived Thursday in St. Petersburg.
Trying to counter the impression of isolation, the White House arranged for a joint statement including those allies as well as Australia, Britain, Italy, Japan, Spain and South Korea condemning the Aug. 21 chemical weapons attack in the suburbs of the Syrian capital, Damascus. According to U.S. intelligence agencies, the attack killed more than 1,400 people.
"We call for a strong international response to this grave violation of the world's rules and conscience that will send a clear message that this kind of atrocity can never be repeated," the statement said. "Those who perpetrated these crimes must be held accountable."
The statement did not explicitly endorse military action, and some of the signatories, such as Italy, have warned against a U.S. strike. But Obama administration officials argued that those that signed understood that they were backing the United States as it was preparing for military retaliation, and therefore effectively embracing it.
Speaking with reporters before returning to Washington, Mr. Obama repeatedly declined to say whether he would abide by the congressional vote if lawmakers say no to his request to authorize the use of force against Syria. He acknowledged facing a difficult task to persuade Congress. "I knew this was going to be a heavy lift," he said. "I was under no illusions when I embarked on this path. But I think it's the right thing to do. I think it's good for our democracy. We will be more effective if we are unified going forward."
The Syria dispute came to dominate the G-20 meeting, often focused mainly on economic matters, and underscored the difficulty Mr. Obama has faced with Mr. Putin in recent months. After Russia gave temporary asylum to Edward J. Snowden, the National Security Agency contractor who disclosed secret U.S. surveillance programs, Mr. Obama canceled a separate one-on-one meeting with Mr. Putin in Moscow.
But the two ended up talking on the sidelines of the group session Friday, mainly about their disagreement over Syria. Mr. Obama said Mr. Snowden's case did not really come up. "It was a candid and constructive conversation, which characterizes my relationship with him," Mr. Obama said.
For his part, Mr. Putin said the two leaders agreed to disagree during a friendly encounter Thursday that lasted more than 20 minutes. He added that they did agree that Syria ultimately needed a political settlement, and that they delegated the matter to Russia's foreign minister, Sergei V. Lavrov, and Secretary of State John Kerry. In May, the two officials announced an effort to begin negotiations for a settlement in Syria, to be held in Geneva, but that effort has since stalled and now seems further away than ever.