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I doubt Obama meant he would unilaterally "ban" earmarks. I just see him using his bully pulpit to advocate for one of his priorities. I also doubt, as Retzilian already pointed out, that he personally views the issue as simply as the campaign talking point suggests.
One of the things he said he would do is make govt. more transparent, and I would be all for any reasonable efforts he makes in that direction, even if only for the Legislative. Wasn't it Obama that either came up with or highlighted the 'google for govt.' idea? I think banning earmarks might, might, encompass far more than simply banning earmarks. I saw Obama stress the idea of the avg. Joe being able to look at exactly where all the federal money goes and who sent it there, and if that idea is realized it would be quite a sea change.