President Obama remains optimistic that Congress will approve a war resolution giving him the authority to strike militarily against Syria despite mounting opposition to military action on Capitol Hill, Obama Chief of Staff Denis McDonough said Sunday.

"I have every confidence we will be able to get this," McDonough said on CBS' "Face the Nation."

But even top lawmakers allied with the president have doubts that the resolution will pass when Congress takes it up this week.

"It's an uphill slog from here," said House Intelligence Committee Chairman Mike Rogers, R-Mich., adding it's "very clear he has lost support in the past week."

McDonough, who made the rounds on the Sunday morning talk shows, said the administration continues "working this very hard," briefing more than 100 lawmakers on the need to retaliate against Syrian President Bashar al-Assad's regime after it used chemical weapons against its own citizens.

Congress returns from August recess Monday and is preparing for a vote on the war resolution as early as this week.

McDonough said that in meeting administration officials have had with lawmakers over the past week that no one doubts that the Assad's regime used the chemical weapons to kill more than 1,400 of its own people.

"Not a person is rebutting or refuting the intelligence," McDonough said on Fox News Sunday. "The question for Congress to resolve this week is are there consequences for a dictator who uses those weapons to gas to death its people including hundreds of children."

Still, with the public widely opposed to military intervention in another Middle Eastern country, informal vote tallies show the vast majority of lawmakers against it or leaning toward opposing a strike.

Obama also lacks the backing of the United Nations for a strike. Adding to the uncertainty Sunday, Secretary of State John Kerry said that Obama has not ruled out returning to the U.N. Security Council seeking support for a strike once U.N inspectors complete their investigation in Syria.

Obama had earlier dropped plans to seek U.N. approval when the security council hit a deadlock because of opposition from Russia and China to any strikes on Syria.

Kerry made the remarks during a news conference in Paris, where he is meeting with his counterparts from the Arab League to seek their support for the strike.

To bolster the case for intervention, the White House and its congressional backers this weekend released a new video they say depicts the horrific aftermath of Assad's Aug. 21 chemical weapons strikes in the suburbs of Damascus.

"I hope that every member of Congress, before he or she casts a vote, will take a look at those videos," McDonough said on Fox.

Sen. Rand Paul, R-Ky., who has emerged as a leading opponent to a strike, said on Fox that he will insist that Senate leaders permit a vote on an amendment that would prohibit the president from proceeding with a military strike if Congress does not authorize it.

"A filibuster can temporarily delay a vote but it cannot put off a vote," Paul said.

Obama has not ruled out attacking on his own authority if Congress does not support him. McDonough on Sunday dismissed such speculation as hypothetical but highlighted the administration's belief a strike was needed not just to punish Syria but to "make sure the Iranians, Hezbollah and others understand you cannot have greater operating space for weapons of mass destruction, like the nuclear program in Iran."

The president will address the nation Tuesday night in last-minute bid to win over an American public that is flooding lawmakers' phones with calls against military action.

Rep. Justin Amash, R-Mich., appearing on "Face the Nation," said his constituents are expressing "overwhelming disapproval" for a strike and he does not believe Obama's Tuesday address will sway an increasingly opposed Congress.

"It's highly doubtful," Amash said.

Appearing on the same show, Rep. Elijah Cummings, D-Md., said the president has not yet made a compelling case for a strike, "But I think he could possibly get the votes," if he did so.

Cummings said "it's possible," he will support the strike. "I want to support the president."