WASHINGTON — President Obama abruptly changed course on Saturday and postponed a military strike against the Syrian government in retaliation for a chemical weapons attack so he could seek authorization first from a deeply skeptical Congress.

In one of the riskiest gambles of his presidency, Mr. Obama effectively dared lawmakers to either stand by him or, as he put it, allow President Bashar al-Assad of Syria to get away with murdering children with unconventional weapons. By asking them to take a stand, Mr. Obama tried to break out of the isolation of the last week as he confronted taking action without the support of the United Nations, Congress, the public or Britain, a usually reliable partner in such international operations.

“I’m prepared to give that order,” Mr. Obama said in a hurriedly organized appearance in the Rose Garden as American destroyers armed with Tomahawk missiles waited in the Mediterranean Sea. “But having made my decision as commander in chief based on what I am convinced is our national security interests, I’m also mindful that I’m the president of the world’s oldest constitutional democracy.”

Although Congressional leaders hailed his decision to seek the permission of lawmakers who had been clamoring for a say, the turnabout leaves Mr. Obama at the political mercy of House Republicans, many of whom have opposed him at every turn and have already suggested that Syria’s civil war does not pose a threat to the United States. His decision raises the possibility that he would be the first president in modern times to lose a vote on the use of force, much as Prime Minister David Cameron of Britain did in Parliament last week.