This won't be a standard party-line vote. Big factions within both parties remain skeptical about President Obama's plans to launch punitive airstrikes against Syria.
If the vote were held today, it might not pass. Obama and his allies — including top House leaders of both parties — have a big selling job yet to do to persuade a majority of members to authorize military action.
Not to mention the public. A flurry of polls has shown that few Americans support a new bombing campaign in the Middle East.
What makes this particularly difficult is the need to get the authorization language just right — tough enough to preserve support among those who favor an aggressive stance against Bashar Assad's regime in Syria, without making it sound too bellicose to drive away those nervous about bombing leading to a broader U.S. engagement.
Here are some of the various congressional factions that Obama will have to contend with:
War-Weary Democrats
Democratic House members such as Steve Cohen of Tennessee and Rick Nolan of Minnesota have publicly described themselves as "war weary." After more than a decade of war in Iraq and Afghanistan, they want to avoid casualties and prefer to spend U.S. funds at home.
In a letter to Obama, Rep. Kathy Castor of Florida warned than an overt military strike "is likely to exacerbate violence in the Middle East."
Those opposed to the use of force could make up a sizable percentage of the Democratic House caucus. Two years ago, 70 Democrats voted against authorizing force in Libya. In July, 111 voted for an amendment that would have blocked the National Security Agency from freely collecting phone data on Americans.
"There's maybe a third of the Democratic caucus who, even when the administration has been supportive of the use of force, have been skeptical," says an aide to one House Democrat.
Tea Party Isolationists
Some Republicans reject the " isolationist" label when it comes to this vote, saying they don't see the value of taking sides in another country's civil war. Senators such as Rand Paul of Kentucky and Mike Lee of Utah have said there's no vital interest at stake for the U.S. in Syria.
Many have also questioned Obama's handling of national security issues in general.
"I don't think the president has met the burden with members of Congress or the American people in terms of laying out what exactly a limited strike would achieve and what the way forward is," says GOP Rep. Tim Griffin of Arkansas. "I'm not convinced it's in our national interest."
Griffin promises to keep an open mind during the coming days of debate, but he says his constituents are almost unanimously opposed to such an operation. "I've heard most of the arguments, I think, and I'm not convinced," he says.
At least in the House, Republican doubters might make up a majority of the caucus.
"Even if you give a limited OK, is there the potential for that to be drawn out into something bigger?" asks Doug Sachtleben, spokesman for GOP Rep. John Fleming of Louisiana, who is against authorizing the use of force.
International Norm Upholders
No matter their queasiness about getting the U.S. involved in another war, numerous members of Congress are also troubled by Syrian use of chemical weapons. They can be persuaded to support punitive strikes — as long as such action is strictly limited.
Democratic Rep. Jim Langevin of Rhode Island, for instance, says he's worried that letting the Aug. 21 gas attack go unanswered will "give the green light" to other nations and terrorist groups.
But he's wary of war. Langevin will only support strikes that are "very targeted, with limited scope and duration," says his press secretary, Meg Fraser. "He's been very strong on that — he does not want to see boots on the ground."
This is Obama's most potentially fertile territory, with lots of Democrats and Republicans alike ready to punish Assad over chemical weapons but not wanting to get drawn deeply into the conflict.
Interventionists
Almost since the start of the Syrian conflict in 2011, some members of Congress, mostly on the Republican side, have urged that the U.S. play an active military role.
GOP Reps. Mike Pompeo of Kansas and Tom Cotton of Arkansas — both veterans — say that Obama should have acted earlier against Assad.
"Inaction will tell Assad, Kim Jong Un [of North Korea] and others that it's open season for the use of chemical weapons," they wrote in The Washington Post. "If we won't act against a use of weapons of mass destruction, Iran will surely believe that we will not act against its nuclear program."
At this juncture, some members of Congress — notably GOP Sen. John McCain of Arizona — want Obama to do more than punish Assad. The U.S. must also "degrade" the Syrian regime's killing capabilities and help the rebels overthrow him.
One way to characterize it: If you're going to go in, go in big. On Wednesday, McCain successfully amended the Senate Foreign Relations Committee's draft resolution to state that it would be U.S. policy "to change the momentum on the battlefield in Syria."
"I'm trying to reconcile why, if we're going to go in there militarily, if we're going to strike, why wouldn't we try to do some kind of knockout punch?" Wisconsin Republican Sen. Ron Johnson said at Tuesday's Foreign Relations Committee hearing.
Johnson voted against the resolution approved by the committee on Wednesday. Other Republican members have questioned the value of "pinprick strikes" that wouldn't destroy Syria's chemical weapons stocks or tip the balance of power.
But pleasing them by taking on a more expansive mission could cost Obama more votes among congressional skeptics than he'd be likely to gain.
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