In retrospect, the bigger surprise of Mary Landrieu’s political career may not have been her defeat Saturday in her bid for a fourth term in the U.S. Senate from Louisiana, but her victory in the Senate race six years ago.

As it has turned out, that 2008 win by Landrieu was the last rung up by a Democrat in a statewide election in Louisiana. That same year, Republicans won six of the state’s seven U.S. House seats. John McCain, the Republican nominee for president, captured 59 percent of the Louisiana vote.

By then, the state’s other U.S. Senate seat was held by a Republican, David Vitter, whose election in 2004 marked the first time Louisiana voted in a Republican senator since the 1870s. The majorities in both houses of the state Legislature were on the way to turning from Democratic to Republican because of legislators switching parties.

Landrieu bucked the strong Republican tide in 2008, taking 52 percent of the vote to win outright in the November primary, as Democrat-turned-Republican John Kennedy finished second with 46 percent. She was helped by the presence at the top of the ballot of Democrat Barack Obama, who would win election as the first African-American president and who helped drive a sizable turnout of black voters that gave Landrieu near-unanimous support.

But the key to her success was her ability to attract nearly a third of the white vote, according to exit polls. That gave her 205,000 more votes than Obama received in the state.

“Looking back at 2008, it’s clear that having Obama on the ticket was important for Landrieu,” said Pearson Cross, who teaches political science at the University of Louisiana at Lafayette. “It was also important that John Kennedy wasn’t truly accepted by Republicans as a legitimate bearer of the party standard.”

Republican voters did not all rally around the Saturday winner, U.S. Rep. Bill Cassidy, of Baton Rouge, in the first round of voting Nov. 4, when eight candidates appeared on the ballot. Landrieu edged Cassidy, 42 percent to 41 percent, but tea party Republican Rob Maness drew 14 percent of the vote.

With no candidate winning the majority needed for election outright on Nov. 4, the top two finishers — Landrieu and Cassidy — met in a runoff Saturday. Cassidy, as the lone Republican in that matchup, unified the state’s now well-established Republican electoral majority behind him and won easily, 56 percent to 44 percent.

“The election of Bill Cassidy in essence cements a Republican future in Louisiana for the foreseeable future,” Cross said.

“It may take a generation for the Democratic Party in Louisiana to recover from this,” said Brian Brox, a political science professor at Tulane University in New Orleans.

“Senator-elect Cassidy’s win symbolizes completely the end of an era,” said Joshua Stockley, a political scientist at the University of Louisiana at Monroe. “For a long period of time, Louisiana was a Democratic state. It could be counted on to support Democratic presidential candidates, it could be counted to send Democrats to Congress, and it could be counted on to send Democrats to Baton Rouge.”

That Democratic Louisiana was part of the “solid South,” which after Reconstruction was dominated politically by white Democrats; most black citizens were denied the vote, and the few who could vote aligned with the Republican Party. But the Democratic coalition of Northern liberals and Southern conservatives fissured in the civil rights movement, as the national party endorsed black aspirations for equality.

With the exception of Dwight Eisenhower in Louisiana in 1956, no Republican presidential candidate through the 1960 election had carried a Deep South state since the end of Reconstruction in the 1870s. But in 1964, Republican Barry Goldwater won the five Deep South states of Louisiana, Mississippi, Alabama, Georgia and South Carolina while suffering a landslide nationwide defeat. Aside from scattered successes by Jimmy Carter, of Georgia, and Bill Clinton, of Arkansas, no Democratic presidential candidate has taken a Deep South state since.

Black voters today overwhelmingly favor the Democratic Party. But they make up less than a third of the Louisiana electorate — and the white Democratic voter in the South is a vanishing species. No more so than in Louisiana: One in six white voters backed Landrieu on Nov. 4, according to exit polls.

With the defeat of Landrieu Saturday and of Georgia Congressman John Barrow in November, no white Democrat from the Deep South will serve in the 2015-16 Congress.

“We’re seeing the culmination of long-term trends of changes in party support in the Deep South,” Brox said.

“We now have an almost completely racialized political party system,” Cross said, with white Southerners solidly voting Republican and black Southerners even more solidly voting Democratic. “It’s the complete reversal of 1945,” he said.

But the issues go beyond race, Cross said.

“The values that are pre-eminent for Republican Party voters are probably freedom, independence, autonomy, limited government and self-reliance, and a strong belief in the powers of capitalism and a free market,” Cross said. “We see them in the South: the calls for less government, the call for people to take personal responsibility, the inveighing against the dependent class.”

Social issues are less significant, Cross said, with the possible exception of reproductive rights. And in Louisiana and other energy-producing states, he said, the perceived antipathy of national Democrats for the oil and gas industry helps swell Republican ranks.

“We’re looking at something more than just a bad year,” said Charlie Cook, the Shreveport native who writes the Cook Political Report in Washington, which tracks congressional politics.

“Democrats are effectively losing everything they could lose in the South,” retaining at the federal level only U.S. House seats in black-majority districts, such as the one represented by Cedric Richmond, of New Orleans, Cook said.

“Democrats are becoming a party of big cities and college towns, and not much beyond that,” Cook said. “For whatever reason, noncollege-educated white voters are looking at the Democratic Party and they don’t see it as representing them.

“These people are kind of a no-fly zone for the Democratic Party.”

Because of the distribution of population across the country, the Democrats remain in a strong position when it comes to presidential elections. But as the makeup of the incoming Congress demonstrates, their claim on power in the House and Senate is more tenuous.

“There’s no way for the Democrats to be a national party without the South,” Stockley said. “How do they intend to take back the Senate? How do they intend to cut into the Republican majority in the House of Representatives?

“What are they going to do to win back Southern whites?”

Follow Gregory Roberts, of The Advocate Washington bureau, on Twitter, @GregRobertsDC