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 is on the verge of being realized this weekend.

If Mary Landrieu, a Democratic senator from Louisiana, loses re-election in Saturday’s runoff election, as expected, the Republicans will have 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 would 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 Johnson signed that bill in 1964. And they still held a majority as recently as a decade ago. Landrieu’s defeat would essentially mark 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. 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.

The dramatic decline of the Southern Democrats represents the culmination of a half-century of political realignment along racial and cultural lines. "Some of it is about Obama; most of it is about the longer-term realignment of white voter preferences," said Guy Molyneux, a Democratic strategist. The shift has contributed to the polarization of national politics by replacing conservative Democrats, who often voted across party lines, with conservative Republicans who do not.

Southern Democrats allowed the party to pass sweeping social programs, like the New Deal and the Great Society, even as they opposed civil rights legislation. They allowed Democrats to hold the House and the Senate, even when the party was at a severe disadvantage in presidential elections. The new alignment makes it all but impossible for Democrats to enact their agenda on issues like climate change, immigration and income inequality. It gives the Republicans a real opportunity, despite all of their demographic challenges, to control the House, Senate and presidency after the 2016 presidential election.

In some states, the Republican advantage among white voters is nearly 9-to-1 in presidential elections, a level of loyalty that rivals that of African-Americans for Democrats. What has changed is that Southern white voters are now nearly as hostile to born-and-bred Southern Democrats, like Landrieu, as they were to John Kerry or Barack Obama.

White supremacist Democrats seized control of the South after the end of Reconstruction, the period that followed the Civil War. They instituted so-called Jim Crow laws disenfranchising African-American voters, who favored Republicans, the party of Lincoln. The so-called Solid South all but unanimously supported Democrats for more than half a century, with states like South Carolina and Mississippi routinely offering Democrats more than 95 percent of the vote, even to losing presidential candidates.

The Democratic hold on the South in presidential elections began to change in 1948, when the Democratic National Convention backed President Harry Truman’s position on civil rights. Many Southern Democrats left the convention and nominated Strom Thurmond as the presidential candidate of the States’ Rights Democratic Party.

But Southern Democrats would continue to dominate state and local politics for decades longer, slowly yielding to Republicans only after the enactment of the Civil Rights Act, after which Thurmond switched to the Republicans and became the first senator from the party to represent the Deep South since Reconstruction.

The demise of the Southern Democrat is not coincidental. It reflects a complete cycle of generational replacement in the post-Jim Crow era. Old loyalties to the Democratic Party have died along with the generation of white Southerners who came of age during the era of the Solid South, before Brown v. Board of Education, before the Civil Rights Act.

Yet it also reflects the very specific conditions of 2014. Today’s national Democratic Party is as unpopular in the South today as it has ever been, in no small part because the party has embraced a secular agenda that is not popular in the region.

"It’s a completely different party than it was 20 or 30 years ago," said Merle Black, a professor of political science at Emory University. "When the Democratic Party and its candidates become more liberal on culture and religion, that’s not a party that’s advocating what these whites value or think."

The party is also led by an unpopular president who has never appealed to the region’s white voters. Obama won about 17 percent of white voters across the Deep South and Texas in 2012, based on an analysis of pre-election polls conducted by the Pew Research Center, census data and election results.

It is difficult to determine how much of that was because of race. There is certainly a case that it was a factor: In 2008, Obama fared far worse than John Kerry in many white areas in the South, even though Kerry was also a northern liberal and even though 2008 was a far stronger year for Democrats than 2004.

Yet nonracial factors are most of the reason for Obama’s weakness. The long-term trends are clear: Kerry, for instance, fared worse than Michael Dukakis among most white Southerners, often losing vast swaths of traditionally Democratic countryside where once-reliably Democratic voters had either died or become disillusioned by the party’s stance on cultural issues. It seems hard to argue that the Democrats could have retained much support among rural, evangelical Southern voters as the party full-throatedly embraced liberalism on issues like same-sex marriage and abortion.

What is clear is that today’s Southern Democrats are no match for today’s racial and political polarization. Last month, strong Democratic Senate candidates — with recognizable political family names, the benefits of incumbency and, in some cases, flawed opponents — lost across the region. They struggled to run more than a couple of points ahead of Obama.

Part of the reason may be that the remaining Southern Democrats were not as conservative as their antecedents. The old Southern Democrats used to "mix it up," according to Black, rarely voting with the party more than 80 percent of the time. Democrats like Landrieu voted with the national party nearly all of the time, including for the Affordable Care Act.

The enfranchisement of black voters, who constitute more than 30 percent of the region’s population and overwhelmingly support Democrats, prevents Republicans from matching the total dominance achieved by Democrats during the era of the Solid South. Back then, state legislatures and congressional delegations might be unanimously Democratic; a losing Democratic presidential candidate might receive 97 percent of the vote, as John Davis did in South Carolina in 1924.

But white support for Republicans in the South might rival, or in some places even exceed, white support for Democrats during the Solid South. In the November election, Landrieu received only 18 percent of the white vote, according to the exit polls, a figure nearly identical to the 19 percent of the vote that Republicans averaged in the state’s presidential elections from 1880 through 1948. The exit polls showed that Obama won 14 percent of white voters in Louisiana in 2008.

The demise of the Southern Democrats now puts the party at a distinct structural disadvantage in Congress, particularly in the House. The young, nonwhite and urban voters who have allowed Democrats to win in presidential elections are inefficiently concentrated in dense urban areas, where they are naturally drawn into overwhelmingly Democratic districts by congressional mapmakers. They are also concentrated in populous states, like California and New York, which get the same number of senators as Alabama or Mississippi.

It remains to be seen whether Republicans will continue to fare so well after Obama leaves the White House. Yet a Democratic rebound seems unlikely anytime soon. With Republicans now holding the advantage of incumbency, unless the region’s religiosity dims or the Democrats relent on their full-throated embrace of cultural liberalism, it may be theirs for a generation.

Nate Cohn, New York Times