The administration and members of Congress, all skeptical of Syria's intentions, also looked to the United Nations as the Security Council arranged closed-door consultations on steps against the government of President Bashar Assad in Damascus.
While Obama made his case in person on Capitol Hill, Defense Secretary Chuck Hagel told a congressional hearing there was still a clear need to support the president's call for legislation authorizing a military strike.
"For this diplomatic option to have a chance at succeeding, the threat of a U.S. military action, the credible, real threat of U.S. military action, must continue," Hagel said.
At the same hearing, Secretary of State John Kerry said any diplomacy "cannot be a process of delay. This cannot be a process of avoidance."
He later added that any agreement must include binding consequences if Syria fails to comply, and lawmakers moved quickly to rewrite pending legislation along the same lines.
Obama himself "wasn't overly optimistic about" prospects for a solution at the U.N., said Sen. Richard Durbin of Illinois, the second-ranking Democrat, after his party's rank and file met privately for lunch in the Capitol with the president. He quoted Obama as saying that even if a credible plan could be worked out, it could be difficult to push through the U.N. Security Council.
The president readied his nationwide speech against a unpredictable chain of events stemming from a chemical weapons attack in the Damascus suburbs on Aug. 21 that the Obama administration swiftly blamed on Assad's government.
U.S. officials say more than 1,400 died in the episode, including at least 400 children, and other victims suffered uncontrollable twitching, foaming at the mouth and other symptoms typical of exposure to chemical weapons banned by international treaty. Other casualty estimates are lower, and Assad has said the attack was launched by rebels who have been fighting to drive him from power in a civil war that has so far claimed the lives of more than 100,000 civilians.
Assad's patron, Russia, has blocked U.S. attempts to rally the Security Council behind a military strike. But Monday, after a remark by Kerry, it spoke favorably about requiring Syria to surrender control of its chemical weapons, and the Syrian foreign minister did likewise.
The foreign minister, Walid al-Moallem, said Tuesday that his government was ready to turn over its chemical weapons stockpile in line with Russia's proposal in order "to thwart U.S. aggression." He also said Syria is prepared to implement a Russian proposal to put its chemical weapons arsenal under international control.
Syria has never provided an accounting of the size of its stockpile, rarely referring in public to its existence. According to an unclassified estimate by the French government, it includes more than 1,000 tons of "chemical agents and precursor chemicals," including sulfur mustard, VX and sarin gas
Obama has said frequently he has the authority as commander in chief to order a military strike against Assad regardless of any vote in Congress, and he has consistently declined to say whether he would do so if lawmakers refuse to approve the legislation he is seeking.
The response in Congress to support such a strike has been lukewarm at best — as underscored during the day when liberal Sen. Ed Markey, D-Mass., and conservative Rep. Mark Mulvaney, R-S.C., both announced their opposition.
Markey, who was elected to the seat that Kerry vacated when he joined the Cabinet, said the legislation under consideration was too broad, "the effects of a strike are too unpredictable, and ... I believe we must give diplomatic measures that could avoid military action a chance to work."
Said Mulvaney: "While I am concerned about taking no action, it strikes me that international law cannot be upheld via unilateral attack by the United States."
Yet Maryland Rep. Steny Hoyer, the second-ranking Democrat in the House, said, "It would be inimical to our country's standing if we do not show a willingness to act in the face of the use of chemical weapons and to act in a limited way to address that use alone."
Hours before Obama's speech from the White House's East Room, Hoyer added, " I don't think there's any doubt that failure to do so would weaken our country, create a more dangerous international environment and to some degree undermine the president of the United States."
Earlier, Senate Republican leader Mitch McConnell became the first congressional leader to come out against legislation giving the president authority for limited strikes. "There are just too many unanswered questions about our long-term strategy in Syria," he said.
By contrast, Speaker John Boehner of Ohio and Majority Leader Eric Cantor of Virginia, the top two Republicans in the House, have endorsed Obama's request.
Given the uncertainty of diplomatic maneuvering, no vote is expected for several days, if then.
"If something can be done diplomatically, I'm totally satisfied. ... I'm not a shock and awe guy," said Senate Majority Leader Harry Reid in a reference to the massive display of firepower that opened the war in Iraq nearly a decade ago.
Still, there was ample skepticism in Congress about the United Nations as well as Russia's true intentions, as well as Syria's willingness to be bound by international agreements.
"There is an overwhelming view it would be preferable if international law and the family of nations could strip Syria of the chemical weapons," said Sen. Chuck Schumer, D-N.Y. "And there's a large view we should let that process play out for a little while."
Said Boehner: "Clearly, diplomacy is always a better outcome than military action. But I will say that I'm somewhat skeptical of those that are involved in the diplomatic discussions today."
Associated Press writers Donna Cassata, Alan Fram, Stephen Ohlemacher, Lolita Baldor, Bradley Klapper, Nancy Benac, Deb Riechmann and Matthew Lee in Washington contributed to this story.
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