Obama has asked Congress to approve limited strikes on Syria to punish the Bashar Assad regime for gas attacks on its citizens.
After a meeting at the White House, Sen. John McCain, R-Ariz., said he and Sen. Lindsey Graham, R-S.C., told the president it's been a year since Obama said the use of gas would cross a "red line" bringing a Western response and two years since the president said there must be regime change in Syria.
"We emphasized the importance we place to actions that would degrade Bashar Assad's capabilities [and] upgrade the opposition and to change the momentum on the ground," McCain said. Opposition forces in Syria have been losing exchanges with the Syrian army.
"That does not mean that any of us support even a limited boots on the ground" plan, McCain added.
Graham said, "There seems to be emerging from this administration a pretty solid plan to upgrade the opposition."
Earlier Monday, a Russian official said his country was "not convinced" by the U.S. claims that Syrian President Assad's regime has used chemical weapons.
Foreign Minister Sergei Lavrov dismissed U.S. evidence during an appearance at the Moscow State Institute of International Relations. The Wall Street Journal reported.
"That which our American partners have shown us both in the past and recently ... absolutely has not convinced us," Lavrov said.
"There are no facts, there is just dialogue about 'what we know for sure,'" Lavrov said. "And when we ask for more detailed confirmation, they say, 'You know, it's all secret, so we can't show you.' That means such facts aren't there."
Meanwhile, key lawmakers in the United States said President Obama's resolution to approve bombings against Syria is unacceptable.
"I know it will be amended by the Senate," Judiciary Committee Chairman Patrick Leahy, D-Vt., the Senate's longest-serving member, told reporters after a more than 2 1/2-hour classified briefing that drew 83 lawmakers of both parties and both chambers to the Capitol for a rare Labor Day weekend meeting.
The briefing was conducted by officials from the White House, the State Department, Defense Department and the Office of the Director of National Intelligence.
House Minority Whip Steny Hoyer, D-Md., said after the briefing he "would be shocked" if Congress didn't amend the resolution.
Leahy said the current wording was too open-ended.
Rep. Chris Van Hollen, D-Md., who inspected chemical attacks by Saddam Hussein's regime against Iraqi citizens in the 1980s, said he would push to see a provision in the resolution prohibiting U.S. troops from being sent into Syria and "some time limits" placed on the proposed military action.
Obama has described action as a way of punishing Assad for using weapons the administration describes as nerve gas in an attack that killed more than 1,400 people last month, including more than 400 children.
Sen. Pat Roberts, R-Kan., a former Intelligence Committee chairman, said the administration promised "to come back with a more prescribed resolution."
Even with tighter wording, winning approval of a limited strike will still be "tough going" for the administration, at least in the House, Rep. Tom Cole, R-Okla., said.
"I think it's going to be a very tough sell," said Cole, often a key crossover Republican in White House compromises.
Cole said he was "leaning no" on approving any use of force against Syria.
Rep. Peter King, R-N.Y., a former Homeland Security Committee chairman, went so far as to say the measure would likely lose if a vote were held in the House now, because of the body's "isolationist wing."
Cole added he expected Obama would be bound by the House and Senate votes and wouldn't authorize a strike if Congress voted no.
Lawmakers said the biggest stumbling block was the fear a limited strike would do little to deter Assad but would clearly draw the United States deeper into Syria's civil war.
"I don't think there's a lot of doubt that the regime undertook this attack," said House Intelligence Committee member Jim Himes, D-Conn. "There's a great deal of skepticism that a limited strike is likely to be effective."
Both the House and Senate are expected to debate and vote on the measure shortly after they return from recess Sept. 9.
While a number of lawmakers who spoke Sunday said they were reticent about supporting military action, Senate Majority Leader Harry Reid of Nevada and House Democratic leader Nancy Pelosi of California issued statements strongly supporting it.
Senior aides told The Wall Street Journal the Democratic leaders would work hard to build support within their party.
Sunday, McCain, a key Republican Obama ally in the use of force, said he would not support "isolated military strikes in Syria" that were not part of a broader strategy to shift the momentum on the battlefield to the rebels.
He also questioned the administration's decision to seek congressional approval before acting, pointing to Obama's declaration months ago a chemical attack would be a "red line" that, if crossed, would be met with military force.
"He didn't say that it's a red line and, by the way, I'm going to have to seek the approval of Congress," McCain said on CBS' "Face the Nation."
Before Sunday's congressional briefing, Secretary of State John Kerry said on five morning talk shows he expected Congress would ultimately back the president's plan for military action.
"I do not believe the Congress of the United States will turn its back on this moment," he said on "Meet the Press."
"I think the interests that we have with respect to potential future confrontation, hopefully not, but the challenge of Iran, the challenges of the region, the challenge of standing up for and standing beside our ally Israel, helping to shore up Jordan -- all of these things are very, very powerful interest[s]. And I believe Congress will pass it."
When asked if Obama would act regardless of what Congress votes, Kerry said, "I said that the president has the authority to act, but the Congress is going to do what's right here."
Kerry said on the talk shows the administration now had evidence the chemical weapons allegedly used in the attack contained sarin. He said blood and hair samples from first responders who helped attack victims tested positive for the nerve agent.
Kerry later reached Arab diplomats to garner support ahead of an Arab League meeting in Cairo Sunday night.
The league later said in a statement it held the Assad regime "fully responsible" for the chemical weapons attack and asked the United Nations and the international community "to take the necessary measures against those who committed this crime."