As any historian (or adept middle school social studies student) will tell you, this was not always the case. In September, we presented a chart of every Senate seat in every state since the republic began. We've updated that with the results of this year's contests; it is below. But let's focus on the South for a second.
The white box -- the period that Landrieu's seat was blue -- highlights the dominance of the Democrats in the South for most of the 20th century. There were a few times that seat (and the others) were vacant, but when re-filled, they were filled with Democrats. No longer.
Of the states that once comprised the Confederacy (notice that most didn't have senators for a few years after 1860), only two still have Democratic senators: Florida and Virginia, increasingly Democratic thanks to migration into its northern suburban counties.
And, without further ado, here's that complete history. Run your eye down the far right side, representing the most recent election. A lot of blue-to-red -- but also a lot of once-split states now having single-party representation.