> It was lost because Obama is a clean, articulate, light-skinned person of color.
The problem with this analysis is that it does not explain the results in the losing Senate races.
That's explained by simple coattails. Obama's cult of personality drove his voters to the polls and they voted straight D ticket. Because they are driven solely by their emotion for Obama, there was nothing else there to push them in one direction or another. They identify as Democrats because of their love for Obama. If they would have had to, say, come back on the next day to vote for non-presidential races, they wouldn't have shown up (just as they didn't show up in 2010). But it's just natural to vote straight-party for the party of the guy they adore.
In states where there was a party mismatch between the presidential winner and the Senate winner, there were probably other issues in play, as in Missouri and Indiana and those Senate candidates' gaffes.