Members of Congress, many of whom are loathe to repeat the quagmire that was Iraq, face mounting pressure in their home districts where they fear a backlash at the polls, particularly in party primary votes next year.
Obama's administration has spent the last week intensely lobbying, but it has made little headway amid extraordinary bipartisan divisions over Syria, as lawmakers question military objectives and long-term goals of a limited strike, and worry about expansion of regional instability as a result.
They have heard the president's call for action, and many have attended briefings and weighed classified evidence of use of chemical weapons by Syrian President Bashar al-Assad's regime.
And despite universal revulsion in Congress for the deadly attacks last month, if the vote to authorize force were held Friday "I have to believe it would be rejected," congressman Kevin Cramer told AFP.
"I think for many, absent of being convinced, 'no' is going to be the answer" on Syria intervention, Cramer said, noting that the opposition cuts across party, geographical and socio-economic lines.
Congressional leadership aides conducting informal vote counts say nearly 80 percent of House Republicans are to some degree opposed to military intervention, while Democratic support is in the low dozens, Politico reported Tuesday.
According to a Washington Post survey, 224 of the current 433 House members were either "no" or "leaning no" on military action as of Friday. A large number, 184, were undecided, with just 25 backing a strike.
The Senate is expected to vote next week on allowing a limited attack, while the House is due to vote within the next two weeks, according to Republican leader Eric Cantor.
Some Obama backers expressed confidence that colleagues on both sides of the aisle will step into line.
"The more people read from the intel reports and the more they hear from the president, the more supportive they'll be," veteran House Democrat Sandy Levin told reporters as he headed into a closed-door briefing in the US Capitol.
But back home, lawmakers are finding voters congealing in opposition.
"My constituents do not want it, they're very reluctant," said House Democrat Rosa DeLauro, who has been a loyal Obama footsoldier but said she remains undecided about the use of force in Syria.
Cramer hosted a town hall meeting Friday in his state of North Dakota, where he said folks "look you in the eye" and don't hesitate to speak their mind.
"They are adamantly opposed to US involvement... being sucked into a long, drawn-out process," he said. "To the degree anyone is for it, they aren't saying."
Poll after poll highlights the skepticism. A Gallup survey Friday showed 51 percent of Americans oppose strikes in Syria compared to 36 percent in favor, a larger opposition ratio than before the onset of the Gulf War of 1991, Kosovo (1999) Afghanistan (2001) and Iraq (2003).
Lawmakers have been keeping tallies of constituent support for and against military force, and the vast majority of those phoning or emailing their representative are opposed.
Republican congressman Andy Harris's office said the count Friday stood at 1,187 constituents against and 27 in favor.
And those firmly behind military intervention, like Senator John McCain, have found it difficult to convince voters.
"The majority of people there are opposed, (and) most of their skepticism is based on Iraq and Afghan experiences," McCain said, after some constituents at a town hall he hosted in Arizona berated the veteran senator for his position.
Republicans who don't want to be seen as associated with backing Obama are also worried.
"In most cases the primary elections are less than a year away, and so candidates are looking over their shoulder wondering if they're going to be challenged within their own party if they take a position in support of the president here," former Republican senator Jon Kyl told AFP.
Some congressional veterans believe lawmakers should look past their constituents' apprehension.
"It weighs on me, no question," Senator Dianne Feinstein, chair of the powerful Senate Intelligence Committee, said of the "overwhelmingly negative" responses coming from her California constituents.
But "they don't know what I know," she said, referring to classified intelligence that convinces her Assad used chemical weapons.