STRELNA, Russia — President Obama raced home on Friday to confront one of the biggest tests of his presidency as he ramped up a campaign to persuade Congress to support airstrikes against Syria that many world leaders he had consulted declined to back.

After two days of tense discussions, including a dinner debate that went into the morning hours, Mr. Obama left without forging 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 on blaming Syria’s government for a chemical weapons attack and on endorsing an unspecified response.

The deep divisions on display here at the Group of 20 summit meeting raised the stakes even further for Mr. Obama as he seeks authorization from Congress for a “limited, proportional” attack. While aides said he never expected or sought a more explicit endorsement of military action during the meeting, the president 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 wasted little time vaulting back into the domestic debate as he called members of Congress from both parties from Air Force One on his way back to Washington. He ordered aides to fan out in coming days with a series of speeches, briefings, telephone calls and television 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 from the White House on Tuesday evening to lay out his case before Congress votes.