If he wants to win in November, Obama needs to avoid "preaching to the choir" mistake that sank Goldwater in '64 and McGovern eight years later:
Both swept their party's nomination thanks to fervent support within the party. Both named running mates who were ideologically similar. Both continued to preach the same message to the general population that they preached in the primaries. And both were defeated in landslides.

Both Obama and Clinton got millions of votes from fervently supported Democrats. But, if they were running mates and held onto every one of those votes in November, it wouldn't be enough to beat McCain. Obama needs to reach out beyond the primary voters to reel in those who didn't vote for him--either because they were for another Dem, or because they are GOP or Independent and are likely McCain voters. That's why he needs a fresh face on his ticket. I think he should choose a white male from the middle or middle-right of the Democratic Party who has defense and foreign policy credentials.