Here's the thing. Barack Obama isn't as popular now as he was in January 2009. This is not exactly a little-known fact; indeed, we fortunately have some really good indicators of exactly how popular Obama is overall, and they're not all that obscure.
What this means is that sloppy journalists can get endless mileage from picking out any subgroup in the nation and finding out that, gee, Obama has lost popularity there! See, for example, a NYT story over the weekend noting that some Obama '08 donors are less enthusiastic this time around. Now, Seth Masket isn't sure that the trend in the story exists in the first place, but even if it does: of course some past Obama supporters don't like him as much now! He's less popular than he was then!
To know whether any of these stories is actually news, it's absolutely necessary to compare Obama's decline within the group in question to his overall decline. If it's more, then you have something; if it's the same or less, then you're at beset illustrating how an overall decline works within that subgroup (which might be a decent story, as long as you're clear about what you're doing). Context matters, and for changes in Obama's approval rating with any subgroup the obvious context is his overall approval ratings.