On Friday night, the president made a last-minute decision to consult lawmakers.
What will happen if they vote no?
It's unclear. A senior administration official told CNN that Obama has the authority to act without Congress -- even if Congress rejects his request for authorization to use force.
Obama on Saturday continued to shore up support for a strike on the al-Assad government.
He spoke by phone with French President Francois Hollande before his Rose Garden speech.
"The two leaders agreed that the international community must deliver a resolute message to the Assad regime -- and others who would consider using chemical weapons -- that these crimes are unacceptable and those who violate this international norm will be held accountable by the world," the White House said.
Meanwhile, as uncertainty loomed over how Congress would weigh in, U.S. military officials said they remained at the ready.
Reactions mixed to Obama's speech
A spokesman for the Syrian National Coalition said that the opposition group was disappointed by Obama's announcement.
"Our fear now is that the lack of action could embolden the regime and they repeat his attacks in a more serious way," said spokesman Louay Safi. "So we are quite concerned."
Some members of Congress applauded Obama's decision.
House Speaker John Boehner, Majority Leader Eric Cantor, Majority Whip Kevin McCarthy and Conference Chair Cathy McMorris Rodgers issued a statement Saturday praising the president.
"Under the Constitution, the responsibility to declare war lies with Congress," the Republican lawmakers said. "We are glad the president is seeking authorization for any military action in Syria in response to serious, substantive questions being raised."
More than 160 legislators, including 63 of Obama's fellow Democrats, had signed letters calling for either a vote or at least a "full debate" before any U.S. action.
British Prime Minister David Cameron, whose own attempt to get lawmakers in his country to support military action in Syria failed earlier this week, responded to Obama's speech in a Twitter post Saturday.
"I understand and support Barack Obama's position on Syria," Cameron said.
An influential lawmaker in Russia -- which has stood by Syria and criticized the United States -- had his own theory.
"The main reason Obama is turning to the Congress: the military operation did not get enough support either in the world, among allies of the US or in the United States itself," Alexei Pushkov, chairman of the international-affairs committee of the Russian State Duma, said in a Twitter post.
In the United States, scattered groups of anti-war protesters around the country took to the streets Saturday.
"Like many other Americans...we're just tired of the United States getting involved and invading and bombing other countries," said Robin Rosecrans, who was among hundreds at a Los Angeles demonstration.
Syria's government unfazed
After Obama's speech, a military and political analyst on Syrian state TV said Obama is "embarrassed" that Russia opposes military action against Syria, is "crying for help" for someone to come to his rescue and is facing two defeats -- on the political and military levels.
Syria's prime minister appeared unfazed by the saber-rattling.
"The Syrian Army's status is on maximum readiness and fingers are on the trigger to confront all challenges," Wael Nader al-Halqi said during a meeting with a delegation of Syrian expatriates from Italy, according to a banner on Syria State TV that was broadcast prior to Obama's address.
An anchor on Syrian state television said Obama "appeared to be preparing for an aggression on Syria based on repeated lies."