Updated to reflect results of Saturday’s runoff election.

After President Lyndon Johnson signed the Civil Rights Act of 1964, he reportedly told a fellow Democrat that the party had lost the South for a long time to come. It took more than a generation for old Southern loyalties to the Democrats to fade, but that vision was realized this weekend.

As Mary Landrieu, a Democratic senator from Louisiana, lost re-election in Saturday’s runoff election, as expected, the Republicans vanquished the last vestige of Democratic strength in the once solidly Democratic Deep South. In a region stretching from the high plains of Texas to the Atlantic coast of the Carolinas, Republicans control not only every Senate seat, but every governor’s mansion and every state legislative body.

Democrats held or controlled nearly every one of them when Mr. Johnson signed that bill in 1964. And they still held a majority as recently as a decade ago. Ms. Landrieu’s defeat essentially marked an end to the era of the Southern Democrats: the conservative, Southern, white officials, supported by white Southerners, whose conflicted views helped define American politics for half a century.

Today, nearly all of the Democrats holding federal or statewide office in the South will represent so-called “majority-minority” districts or areas with a large number of new residents from outside the region. In the states of the former Confederacy, Democrats will control Senate seats or governors’ mansions only in Virginia and Florida. Not coincidentally, those are the two Southern states where people born outside the state represent a majority of the population. These Democrats bear little resemblance to the Southern Democrats who won by attracting conservative white voters.