As the White House now considers direct military action in Syria, something it has resisted for two years, Speaker John A. Boehner wrote a letter on Wednesday to Mr. Obama asking the president to provide a “clear, unambiguous explanation of how military action — which is a means, not a policy — will secure U.S. objectives and how it fits into your overall policy.”

The discussion has even brought in former officials intimately involved in making the hurried public case for the Iraq war. In an interview with Fox Business Network, Donald Rumsfeld, who was defense secretary at the time, said Wednesday that “there really hasn’t been any indication from the administration as to what our national interest is with respect to this particular situation.”

Senator John McCain, the Arizona Republican, has been scathing in his criticism of Mr. Obama for the opposite reason — that the president in his view has not taken enough action. Mr. McCain has said that doubts about military action expressed by the chairman of the Joint Chiefs of Staff, Gen. Martin E. Dempsey, have emboldened the Syrian government to use chemical weapons and that Mr. Obama, having allowed Mr. Assad to cross his “red line” on the use of these weapons on previous occasions, had little standing now.

“Now this is the same president that two years ago said that Bashar Assad must leave office, and so where is America’s credibility?” Mr. McCain said on Fox News. “Where is our ability to influence events in the region? And I promise you that those who say we should stay out of Syria do not understand that this is now a regional conflict.”

The administration plans to brief leaders in the House and Senate with a classified version of its intelligence assessment about the attack, according to Congressional aides.

Americans over all have been skeptical about the United States getting involved in Syria’s civil war, although surveys show they are more open to a limited strike on Syrian targets using cruise missiles or drones.

There has not been a major poll released since last Wednesday’s chemical attacks, but a poll published by Quinnipiac University last month found that 61 percent of people said it was not in the national interest to intervene in Syria, while 27 percent said it was. By a similar split, 59 percent opposed providing weapons to rebel forces, while 27 percent were in favor. But 49 percent of people said they would support missile strikes against government forces if the strikes did not endanger American lives, while 38 percent said they were opposed.