With the defeat of Sen. Mary Landrieu in Louisiana, not a single Democrat from the Deep South will hold a Senate seat come January. That stark fact represents the last chapter of a long story that has been altering American politics for more than a half-century.Two other white Southern Democrats, Kay Hagan of North Carolina and Mark Pryor of Arkansas, also lost their Senate seats. And in Georgia, John Barrow -- the last white Southern Democrat in the House of Representatives -- was ousted as well.When we started covering Congress in the early 1980s, the Senate was still full of Southern Democrats.