It was a curious tactic, given that it was hard to see 1) what evidence they had that would make Assad look worse than he already looks, but still 2) not be bad enough to call for regime change. Well, the answer seems to be, none.
And from its right:
Indeed, news reports are replete with anecdotes of congressional offices being flooded with calls against intervention. I'd be willing to be $100 that the next round of polls will show support for intervention slipping. What I thought was a done deal—congressional rubberstamping of the president's Syria plans—now appears so dead that it would surprise me if they hold a vote in the House at all. (Though Republicans looking to embarrass President Barack Obama would have every motivation to do so.)
But there's another calculation at play that bears discussing: the effects of gerrymandering. Republicans crammed so many House Democrats into so few districts, that the vast majority of Democratic seats are VERY Democratic. So those Democrats don't have to worry about their general elections. But with anti-war sentiment running high in progressive circles, particularly in these primarily urban districts, primary challenges are a possibility. And if things go downhill in Syria, those who voted for the war authorization would almost be guaranteed challenges.
A "no" vote is a safe vote. If things go poorly, they voted no. If things go well, no one will punish them for opposing the war.
A "yes" vote, on the other hand, is fraught with risk. And sometimes, those risks are worth taking. But few could argue that this is one of those times. Most Americans are well aware of our limits, both of our military power, and of our ability to pay for it. They may not fully understand Syria's internal dynamics (who can?), but they understand enough to know it's an epic clusterfuck without any potential favorable outcomes.
So why would any Democrat who isn't already sold on intervention change their vote? It would just open them up to 2014 headaches. And no one needs those.