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They are finding an increasingly skeptical audience on Capitol Hill.

As of Thursday, 185 members of the U.S. House had signed on to letters demanding that the president come to Congress with a full case for military action.

One of those letters, signed by 140 lawmakers, mostly Republicans, said the president had a duty to consult. The other letter, signed by Democrats, said he must obtain permission from Congress first.

“While the ongoing human rights violations and continued loss of life are horrible, they should not draw us into an unwise war — especially without adhering to our own constitutional requirements,” the Democrats said in their letter.

A HuffingtonPost/YouGov poll taken this week found little support for military action: Just a quarter of adults surveyed said the U.S. should launch airstrikes to aid the rebels. Another 41 percent were opposed, and 34 percent were unsure.

A majority of those surveyed, however, do believe Syrian troops used chemical weapons against the rebels.

The congressional Democrats’ letter urged Mr. Obama to let the U.N. inspectors finish their work and told the president to continue pushing for U.N. approval of any further action.

The Obama administration, though, has ruled out that option, saying Russia’s opposition — the Kremlin holds a veto at the Security Council, which would have to approve any authorization to use force — is too big of a hurdle.

U.N. Secretary General Ban Ki-moon has pleaded with Western nations to let U.N. weapons inspectors in Syria finish their work. The inspectors are investigating various sites where rebels say chemical weapons attacks took place, including in the Damascus suburbs where troops loyal to Mr. Assad shelled rebels and civilians last week.

Reports say hundreds were killed in the attack, and photos and video afterward showed dead children, covered in white shrouds, lined up shoulder to shoulder, without any visible evidence of trauma. The U.N. inspectors are expected to finish their work by the end of the week.

Syria and its ally Russia have suggested that it was rebels who deployed chemical weapons.

The Associated Press, citing two unidentified intelligence officials and two other government sources, reported Thursday that the evidence linking the Assad regime to the chemical weapons strike is not a “slam dunk.”

The anonymous officials said a report by the Office of the Director for National Intelligence has key caveats — including an acknowledgment that the U.S. intelligence community no longer has the certainty it did six months ago of where the regime’s chemical weapons are stored, nor does it have proof that Mr. Assad ordered a chemical weapons attack.

Confronted with that report, the White House spokesman discounted the wire service’s use of anonymous sources and said he places more credibility in the assessments of the president, Vice President Joseph R. Biden and others who were willing to go on the record.

“I have on-the-record statements from the president of the United States, the vice president, the secretary of state,” Mr. Earnest said. “I’ve got on-the-record statements from the prime minister of the United Kingdom. We’ve got the president of France. So I leave it to you to decide whether or not you believe anonymous [sources] that are included in the AP story, or an on-the-record statement from people who have looked at exactly the same information.”

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