WASHINGTON — The evidence of a massacre is undeniable: the bodies of the dead lined up on hospital floors, those of the living convulsing and writhing in pain and a declaration from a respected international aid group that thousands of Syrians were gassed with chemical weapons last week.

And yet the White House faces steep hurdles as it prepares to make the most important public intelligence presentation since February 2003, when Secretary of State Colin L. Powell made a dramatic and detailed case for war to the United Nations Security Council using intelligence — later discredited — about Iraq’s weapons programs.

More than a decade later, the Obama administration says the information it will make public, most likely on Thursday, will show proof of a large-scale chemical attack perpetrated by Syrian forces, bolstering its case for a retaliatory military strike on Syria.

But with the botched intelligence about Iraq still casting a long shadow over decisions about waging war in the Middle East, the White House faces an American public deeply skeptical about being drawn into the Syrian conflict and a growing chorus of lawmakers from both parties angry about the prospect of an American president once again going to war without Congressional consultation or approval.

American officials said Wednesday there was no “smoking gun” that directly links President Bashar al-Assad to the attack, and they tried to lower expectations about the public intelligence presentation. They said it will not contain specific electronic intercepts of communications between Syrian commanders or detailed reporting from spies and sources on the ground.

But even without hard evidence tying Mr. Assad to the attack, administration officials asserted, the Syrian leader bears ultimate responsibility for the actions of his troops and should be held accountable.

“The commander in chief of any military is ultimately responsible for decisions made under their leadership,” said the State Department’s deputy spokeswoman, Marie Harf — even if, she added, “He’s not the one who pushes the button or says ‘go’ on this.”

Administration officials said that communications between military commanders intercepted after Wednesday’s attack provided proof that the assault was not the result of a rogue unit acting against orders. It is unclear how much detail about these communications, if any, will be made public.

In an interview on Wednesday with the PBS program “NewsHour,” President Obama said he still had not made a decision about military action. But he said that a military strike could be a “shot across the bow, saying ‘stop doing this,’ that can have a positive impact on our national security over the long term.”

The bellicose talk coming from the administration is unnerving some lawmakers from Mr. Obama’s party, who are angry that the White House seems to have no inclination to seek Congress’s approval before launching a strike in Syria.

“I am still waiting to see what specifically the administration and other involved partners have to say about a potential military strike, but I am concerned about how effective such an action could be,” said Representative Adam Smith, a Washington Democrat who is the ranking member of the House Foreign Affairs Committee. “I am worried that such action could drag the United States into a broader direct involvement in the conflict.”

Despite the Obama administration’s insistence that the graphic images of the attack go far in making a case for military action in Syria, some experts said that the White House had its own burden of proof.

Anthony H. Cordesman of the Center for Strategic and International Studies said that whatever evidence the administration put forward would be the American intelligence community’s “most important single document in a decade.”

The Obama administration, Mr. Cordesman said, needs to use intelligence about the attack “as a key way of informing the world, of building up trust in U.S. policy and intelligence statements, and in moving U.S. strategic communications from spin to convincing truth.”

And yet it appears that the public presentation of the Syria evidence will be limited. Instead of the theater of Mr. Powell’s 2003 speech — which included satellite photographs, scratchy recordings of conversations between Iraqi officials and a vial of white powder meant to symbolize anthrax — American officials said the intelligence assessment they are preparing to make public will be similar to a modest news release that the White House issued in June to announce that the Assad government had used chemical weapons “on a small scale against the opposition multiple times in the last year.”

Based on that conclusion, Mr. Obama authorized a limited program of supplying the Syrian rebels with arms, which have yet to arrive.

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.

It is the fear of the United States getting dragged into yet another Middle Eastern war that before last Wednesday had animated opposition — both inside the White House and across the country as a whole — to American military intervention in Syria.

Even as he now contemplates getting deeper into a war he had long resisted, Mr. Obama appears to be mindful that the opposition remains. “We can take limited, tailored approaches, not getting drawn into a long conflict,” he said Wednesday on “NewsHour.” He added, “Not another repetition of, you know, Iraq, which I know a lot of people are worried about.”