This is going to be different from any vote for military action since World War II. First of all, as I've note in the other thread, there is obviously no UN mandate here. And other than perhaps a few other countries (what are the French contributing - cheese?), we're going to go this one alone in Syria if we go.
One unusual thing about this proposed action compared to other military actions where Congress approved (and they've never said no): we are not offering the Syrian government a way to avoid the violence. When Congress authorized military action in Iraq, both times, Saddam Hussein was offered a way out: get out of Kuwait (1990) or get out of your own country (2003). The military action in Kosovo and Serbia in 1999 continued until Serbia backed down, but they could have acquiesced before it started and not gotten hurt.
This time, it's not "Assad do this or else we kick your ass." Instead, it's "Do we kick Assad's ass or not?" I think that's an important distinction. A congressperson could have voted on Iraq with the belief that the other side still had the opportunity to avoid the violence they are authorizing. Isn't it very different when you authorize violence directly?
On the political side, I wonder if, for Republicans, the desire to embarrass Obama and make him look weak will override their natural inclination to back military action and salute the flag. Congressional Democrats will be split, I believe, as they usually are on matters of military action. And there was a fear (justified or overblown) of Saddam Hussein in the votes over Iraq. I don't think anyone is afraid of Bashar al-Assad.
It's going to be a tough vote. There seems a better than even chance that Obama could lose.