(c) 2013, Bloomberg News.

(c) 2013, Bloomberg News.

WASHINGTON President Barack Obama decided the crisis in Syria, which he called "the worst chemical weapons attack of the 21st Century," required delay rather than action.

In asking for the U.S. Congress to approve the bombing of Syria, Obama reset the notion of presidential authority about war powers that had increasingly resided with the Executive Branch since the end of World War II.

It also represents one of the greatest gambles of his presidency in that he is seeking the approval from a Congress that, at least in the Republican-controlled U.S. House, has fought him at every turn on a wide range of subjects.

Obama's conviction on Saturday, and the moral outrage expressed by Secretary of State John Kerry just the day before, suggest they believe, having seen all of the evidence and not simply the declassified portion that lawmakers have seen, that they will win the debate once Congress is fully informed.

White House officials said the president was shifting the burden to members of Congress, who have been long on criticism and short on solutions as he faces a crowded agenda in the fall that includes the implementation of his health care law. The president, the officials said, wanted to lean into the debate about the role of Congress rather than avoid it.

The authority of the U.S. Senate and House in matters of war was debated intensely before the second conflict in Iraq. Then senators, Vice President Joe Biden, Kerry and Defense Secretary Chuck Hagel all became critics of the war who said President George W. Bush had overstepped his authority. Obama's criticism helped propel his election to the Senate and, eventually, the White House.

In an interview with the Boston Globe in 2007, Obama was asked in what circumstances, if any, a president would have constitutional authority to bomb Iran without seeking a use-of- force authorization from Congress.

"The president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation," Obama said.

"As Commander-in-Chief, the president does have a duty to protect and defend the United States. In instances of self- defense, the president would be within his constitutional authority to act before advising Congress or seeking its consent," he said.

Obama's decision marks a rare moment in the last half century when a president unilaterally decided to give some power back.

"It's quite uncertain what a military strike is going to produce," said Robert Dallek, a presidential historian. "He really shouldn't go it alone. It's wise to bring Congress into it. It gets him much more of a broad consensus."

While "temporarily it would certainly undermine him" if Congress rejects his request, the risk of proceeding without backing is greater, he said. "If you run into difficulties, you're out on a limb. This way he's someone who's acted with political backing," Dallek said.

Lee Hamilton, a Democrat and former chairman of the U.S. House foreign affairs and intelligence committees, said the decision will help Obama "in the country."

"You will have the support of the Congress, which shares the burden. It will help him internationally, which will see the country is united," said Hamilton.

Yet Obama's words almost undercut the sense of indignation that he and Kerry expressed less than 24 hours before, and at the same time put new pressure on Republicans in Congress, who had been urging military force even as they had taken no responsibility for it with a vote. Professional military officers at the Pentagon were also said to be lukewarm about the mission.

Faced with few good options in the short run, Obama chose to pull back so that more time might change the calculus.

Now the administration has the task of persuading Republicans, especially those who represent a more isolationist and Libertarian strain on international affairs, to support a limited military strike that no one argues will end the civil war in Syria, and many have suggested may make it worse.

"It's unfortunate that it's gotten to the point where I have to thank the president for following the Constitution & the law," Rep. Justin Amash, a Republican from Michigan, said in a Twitter posting.

Modern U.S. history makes clear the risk of entering a military conflict without clear backing from Congress, Dallek said. "Harry Truman crosses the parallel in Korea without congressional support. It destroys his presidency. He gets stuck in Korea and ends up with a 32 percent approval rating.

''Lyndon Johnson relies on the Gulf of Tonkin resolution, which was not a blanket commitment to expand and fight in Vietnam, and it essentially destroys his presidency,'' Dallek said.

A request for congressional authorization hadn't been discussed in White House deliberations on a Syria strike until Aug. 30, said two administration officials.

Late in the day, Obama and Denis McDonough, the White House chief of staff, spent 45 minutes walking the South Lawn discussing the idea, with the president laying out the argument that a congressional debate would strengthen him and provide firmer ground for a military strike, the officials said.

When he called aides into his office at 7 p.m. to tell them he had decided to ask for a congressional vote, some dissented, said the officials. They declined to identify the dissenters or detail their arguments.

Obama's relations with Congress have deteriorated almost from the day he has taken office, with House Republicans voting 40 times to either repeal or defund his signature domestic initiative, The Affordable Care Act, along with threatening to shut down the government or even force default on debts.

A force authorization vote in the U.S. Senate would fare better, though two of the strongest Republican supporters for military action, Senator John McCain of Arizona and Senator Lindsey Graham of South Carolina, said they couldn't back the president's call for only limited strikes.

The agenda on Capitol Hill was crowded before the situation in Syria worsened. It must deal with the federal budget, raising the debt ceiling, and it also must consider the president's nominee to be chairman of the Federal Reserve. It also has a proposed revision of immigration laws and other issues of taxes to consider.

Reaction to the president's decision to seek Congress' approval was met with praise from Republicans who also stopped well short of any commitment.

Rep. Howard ''Buck" McKeon, a California Republican and chairman of the House Armed Services Committee, said he "appreciated the president's decision" to consult and agreed that "those who use chemical weapons against their own people should be held accountable." He said the president would need to set out "clear military objectives that can meet articulated policy goals."

By asking Congress to deal with an issue as grave as military action, Obama could be seeking to make matters such as shutting down the government or defaulting on the debt appear petty and trivial.

Obama's calendar is crammed as well. He leaves Tuesday for economic meetings in Sweden and St. Petersburg, Russia, and he will do so now with a chance to persuade other leaders to support him rather than having to defend a decision that already had been rejected in the House of Commons to the embarrassment of Prime Minister David Cameron.

In a week when Obama also helped commemorate the 50th anniversary of the Rev. Martin Luther King Jr.'s "I Have a Dream" speech, the president reprised King's line about the "fierce urgency of now" when he came to dealing with social injustice.

In the case of Syria, he has decided to take the risk of delay, one that could leave him in the same position as Cameron rejected by his own legislative branch at home and seen as weaker abroad.

His decision also puts him more squarely in line with his rhetoric as a U.S. Senator and as a presidential candidate in 2008.

"History has shown us time and again, however, that military action is most successful when it is authorized and supported by the Legislative branch," Obama said in the 2007 Boston Globe interview. "It is always preferable to have the informed consent of Congress prior to any military action."

Julian Zelizer, a presidential history at Princeton University, said that "especially given who he is and how he ran in 2008, the last thing he wants to be is a president acting solo on military action."