WASHINGTON (AP) — President Barack Obama used a nationally televised address Tuesday night to make his case for military action against Syria, even as he recognized that diplomatic steps could render attacks unnecessary. He told war-weary Americans that the use of chemical weapons poses a threat to U.S. security and that America, with modest effort, "can stop children from being gassed to death."

Citing the new diplomatic efforts, Obama said he had asked congressional leaders to postpone a vote on legislation he has been seeking to authorize the use of military force against Syria — a vote he was in danger of losing. But he also said he has ordered the U.S. military to remain prepared to carry out attacks if needed.

He blamed last month's chemical attacks near Damascus squarely on Syrian President Bashar Assad and warned that a failure to act now would encourage tyrants and terrorists to use similar weapons.

"Our ideals and principals as well as our national security are at stake in Syria, along with our leadership of a world where we seek to ensure that the worst weapons will never be used," he said.

Obama's speech was seen as a critical one for his presidency, though for reasons different than when plans for it were announced last week. It was intended as the climax of the administration's pitch to persuade Congress to endorse military action in Syria. Polls showed that Americans, wary of another Middle East conflict, oppose the strikes. Lawmakers from both ends of the political spectrum declared they would vote against the measure.

Syria's announcement this week that it would accept a Russian plan to turn over the chemical weapons stockpile added new uncertainty. Obama's speech was closely watched for signs of how much stock he put in prospects for a diplomatic breakthrough .

He did recognize some potential. Obama noted he was sending his top diplomat, Secretary of State John Kerry, to Geneva for talks Thursday with Russian Foreign Minister Sergey Lavrov. He said he would continue his own discussions with Russian President Vladimir Putin and that the U.S. and its allies would work with Russia and China to secure a U.N. Security Council resolution "requiring Assad to give up his chemical weapons and to ultimately destroy them under international control."

Obama said the Russian initiative "has the potential to remove the threat of chemical weapons without the use of force, particularly because Russia is one of Assad's strongest allies."

But the 16-minute speech generally made the case for military action. His arguments were both practical and emotional.

"If diplomacy now fails and the United States fails to act, he said, "the Assad regime will see no reason to stop using chemical weapons" and "other tyrants will have no reason to think twice about acquiring poison gas and using" it. Over time, he added, the weapons could threaten U.S. troops as well as allies like Turkey, Jordan and Israel.

"America is not the world's policeman," Obama said. "Terrible things happen across the globe, and it is beyond our means to right every wrong. But when, with modest effort and risk, we can stop children from being gassed to death and thereby make our own children safer over the long run, I believe we should act."

The unpredictable chain of events stemmed from a chemical weapons attack in the Damascus suburbs on Aug. 21. U.S. officials say more than 1,400 people died, including at least 400 children, and 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.

Russia has blocked U.S. attempts to rally the U.N. Security Council behind a military strike. But Monday, after a remark by Kerry, Russian officials 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.

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 lukewarm support in Congress was underscored on Tuesday when liberal Democratic Sen. Ed Markey and conservative Republican Rep. Mark Mulvaney both announced their opposition.

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, House Speaker John Boehner and Majority Leader Eric Cantor, the top two Republicans in the House of Representatives, and House Democratic leader Nancy Pelosi have endorsed Obama's request.

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."

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Associated Press writers David Espo, Julie Pace, Donna Cassata, Alan Fram, Stephen Ohlemacher, Lolita Baldor, Bradley Klapper, Nancy Benac, Deb Riechmann and Matthew Lee contributed to this story.