I think there are a few things at play, some of them being: 1. Many in Congress privately like the president taking this power from them. The reason for this is that they do not want to be on record as voting for or against war. 2. Many don't really care so much that the office of the presidency is doing it so much as Obama is doing it. In other words, if their guy was in power they wouldn't be voicing objections. So what they are doing is objecting just to gain political points. 3. Some won't do it because they do not believe it has a chance of having the desired outcome. They don't believe not because their case has no merit but because they believe (and rightly so, imo) that the system is so corrupt that it is impossible to change it.