I had always noticed that W sounded much more Texan when he was making a campaign speech then when he was speaking normally. You can see in his ALS video especially when compared to his Mrs. (George W Bush takes Ice Bucket Challenge for charity - BBC News)
Not living in the US, I usually hear Obama speaking on policy, but I was surprised to hear him speaking on campaign, he sounded much more, I dunno, “folksy”?
Maybe it’s just my foreign ears.