My assumption all along, one I'm still (uneasily) holding to, is that when the debate is over, Congress will give Obama the authority he's asking for to attack Syria, just as it has every other time a president has asked. (There have been a couple of occasions in which Congress voted against a military action, but in those cases the president hadn't actually requested the vote; they were congressional protests against something that had already begun.) But a congressional rebuke, particularly in the House, is starting to look like a real possibility. This is a Congress unlike any that came before it, and the unusual nature of this proposed action—offered mostly as a punishment for something that already happened, with barely a claim that it will do much if anything to stop future massacres so long as they're done with conventional weapons—may combine to set a new historical precedent.
It was pretty remarkable to see Republican members of Congress yesterday yelling at John Kerry about the rush to war like they were a bunch of San Francisco liberals. But for these guys, there's really no higher principle than opposition to Barack Obama and everything he wants to do. And if this is a "conscience" vote (i.e. one where the leadership is not demanding that they toe the party line), a lot of Democrats just don't find the administration's case persuasive.
And for some others, this isn't a difficult vote, it's a golden opportunity. On the left, Alan Grayson is promoting an online petition and encouraging people to contact their representatives (Molly Ball interviews him about it here). On the right, Rand Paul has found the next issue that can keep him in the spotlight and continue the momentum for his 2016 presidential run. And for a guy who really isn't all that experience in politics and seems to know next to nothing about policy, he's been doing a brilliant job turning himself into a media star. As Robert Costa explains in the National Review, "after more than three years of making an often lonely case for less U.S. intervention abroad, this likely 2016 presidential contender finds himself coordinating a brewing conservative rebellion—not only against the Obama administration, but also against his own party's hawks. He's huddling daily with conservatives in both the House and Senate and guiding them on how to battle the leadership."
The Washington Post is keeping a whip count, which at this point in the House has 86 members against, another 92 leaning no, only 19 in favor, and 103 officially undecided (that leaves 135 for which they haven't ascertained an opinion). In other words, the skeptics are much more numerous at this point, but there's plenty of room to assemble a majority in favor of a strike. But what if the House does vote no?
It's hard to imagine the Obama administration will pull back. After all, they've said quite clearly that they believe they don't need Congress' approval, and they will have spent weeks making the case that striking Syria is utterly vital to U.S. national-security interests. It would seem likely they'd go ahead and launch some missiles anyway. As crass as it may be to contemplate, from a political perspective, folks like Grayson and Paul would probably like that just fine—it would give them something to be very publicly outraged about and organize around.