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.

As recently as six years ago, Landrieu polled one-third of the white vote in winning a third Senate term, but her total slipped to 18 percent in the first round of voting last month. She got 94 percent of the black vote, but that wasn’t enough to give her 50 percent overall; she had to face Republican Bill Cassidy in a runoff, and lost badly.

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.

A few African-American Democrats still hold House seats in the old Confederacy, but they represent primarily minority districts and only reinforce a larger point: In the Deep South today, the Republicans are essentially the white majority party and the Democrats are the black minority party.

We admit to a personal interest here. Cokie’s parents, Hale and Lindy Boggs, represented the city of New Orleans in the House for a combined total of close to 50 years. There’s hardly anyone like them left in Congress.

Hale and Lindy were unusual and courageous Southerners, often risking their careers to fight for civil rights legislation. They were also pragmatic professional legislators who helped craft a vast array of productive measures, from creating the federal highway program to mandating equal credit for women.

When we started covering Congress in the early 1980s, the Senate was still full of Southern Democrats who followed Hale and Lindy’s model: Fritz Hollings of South Carolina, John Breaux from Louisiana, Sam Nunn of Georgia, David Pryor of Arkansas.

They were the centrists, the conciliators, the dealmakers who could work across the aisle and lubricate the machinery of compromise. They’re all gone now, largely replaced by hard-right Republicans who think compromise means surrender, if not sedition. It’s instructive that Sam Nunn’s daughter Michelle and David Pryor’s son Mark were both defeated in Senate contests last month.

President Obama clearly hurt Southern Democrats this year ”“ his popularity rating was a dismal 39 percent in Louisiana ”“ but this trend has been gathering steam for a long time.

“Some of it is about Obama,” Democratic strategist Guy Molyneux told The New York Times, but “most of it is about the longer-term realignment of white voter preferences.”

At the core of that realignment is race. It really started in 1948, when President Truman’s decision to integrate the armed forces and oppose poll taxes helped spawn the Dixiecrat rebellion led by Strom Thurmond, then the Democratic governor of South Carolina, who ran for president on a segregationist platform and captured four Southern states.

President Johnson famously predicted that his signing of civil rights legislation in the mid-60s would drive many Democrats into the Republican Party, and he was right. The defectors included Thurmond, by then a senator, who joined the GOP in 1964. Richard Nixon accelerated the shift with his “Southern strategy” appealing to disaffected whites in 1968.

But if race was the biggest factor in this “longer-term realignment” of the South, it was not the only one. As the national Democratic Party embraced issues like abortion rights, gun control and same-sex marriage, many conservative churchgoers felt alienated enough to complain: “I didn’t leave my party; my party left me.”

The Democratic Party today is “a completely different party than it was 20 or 30 years ago,” Merle Black, a political scientist at Emory University, argued in the Times. “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.”

A third factor driving realignment is the tea party’s assault on federal spending. Many Southern Democrats rose to power through the seniority system and solidified their position by funneling federal dollars to their home states. That’s harder to do ”“ and harder to brag about ”“ at a time when Washington is considered a dysfunctional wasteland many voters.

The demise of white Southern Democrats is a tragedy for American politics. The center has been hollowed out. And this vast and diverse nation cannot govern itself effectively without leaders who believe compromise is not a dirty word, but a dire necessity.

— Steve and Cokie Roberts can be contacted by email at [email protected]