It was a NBC/WSJ poll. I seen it on Morning Joe a couple days ago. It was 54-39. I can't seem to find the poll though. Hmmm...
Here is the video from MoJo: http://video.msnbc.msn.com/morning-joe/48735257 (The poll was from July, not August)
But, here it shows Obama with a 10, not 15 point lead: http://firstread.nbcnews.com/_news/2012/08/21/13399788-nbcwsj-poll-heading-into-conventions-obama-has-four-point-lead?lite
(In the margin of error of changing so it's not too crazy, but definitely misleading from Morning Joe!)