The risks of intervention are numerous, and at least some of them are very likely. Political observers who write as if not intervening is the riskier bet can only credibly do so by implicitly adopting a time horizon that ends the minute Congress votes, and Obama either gets his way or does not get his way.

What happens after that?

If Obama loses the vote, it will seem, for a news cycle or three, that he has been damaged, because lots of shortsighted pundits will prattle on and on about the "unprecedented humiliation" of it all. And then, Syria will fade from the headlines. Something else will be the big story. There will be a debt-ceiling fight and midterm elections and debate about whether to give immigration reform another go. The media will start focusing on Edward Snowden's NSA revelations again, Obamacare will start to be implemented, and Politico will declare that some other matter could be a turning point in the Obama Administration.

No future is certain, but that is a far more likely future, if the Syria vote fails to pass, than America spending the next three years thinking that Obama's presidency definitively ended because he wasn't able to start a very unpopular war.