I think that there are MANY things going on here.
On GOP side, you had those who are thinking, "this is the best we'll get and we're gonna get blamed anyway," along with personal interests or beliefs, perhaps (note that even on FR, some are for this).
On the Dem side, you have, "well, the GOP might get blamed overall, but in my district, I'll get creamed if I vote for this piece of thievery!" and "There's not enough pork in this...I'm not going to vote for it until it's fattened up again!" etc.
And, even if I do vote against it, the media will cover my back:
http://www.nytimes.com/2008/09/30/business/30cong.html?hp
Representative Roy Blunt of Missouri, the Republican whip, said that before the vote he had tallied 75 votes in his conference in favor of the plan. By the time the votes were cast, the Republicans could deliver only 65 of them.
Republicans said they had alerted Democrats they might not have the numbers required. But they never recommended the legislation be put off and in the end they were unable to win any last-second converts to change the votes that would have been necessary to turn defeat into victory.