HAMMOND, La.—Mary Landrieu is dead, and everyone knows it but Mary Landrieu.

The senior senator from Louisiana, a diminutive blond woman with a round, youthful face, is standing under a green canopy in the middle of an airfield. The canopy reads, "City of Hammond, Too Lovely to Litter." There is frustration in her voice as she repeats, yet again, the message nobody seems to be hearing. "The national race is over," she says. "This race is clearly now about what's in Louisiana's best interest."

Landrieu's death was foretold on November 4, when any remaining hope Democrats might have had that their candidates' individual qualities could overcome voters' hostility to the president was washed away in a national Republican wave of unexpected proportions. Though Landrieu was also on the ballot that day, thanks to Louisiana's quirky election laws, it was only the first round—an all-parties primary featuring four Democrats, three Republicans, and a Libertarian. Landrieu got 42 percent of the vote; Bill Cassidy, a physician and Republican congressman, got 41 percent. Now the two of them are pitted head-to-head in a runoff election Saturday.

It is the last Senate contest of 2014 to be decided, a lingering loose end, a hangnail of an election that—now that Republicans have already won the Senate majority—will affect practically no one, except Mary Landrieu and her constituents.

Since the primary, Landrieu has undergone a series of humiliations. First, the Democratic Senatorial Campaign Committee announced it would not spend any money supporting her in the runoff. Landrieu's campaign was already practically broke, and in the weeks following the primary more than 90 percent of the television ads Louisiana voters saw were from Cassidy or groups supporting him.

Then, in a last-ditch attempt to demonstrate her clout in the Senate, Landrieu—who describes herself as "a strong supporter of the oil and gas industry"—persuaded Harry Reid, the outgoing Senate majority leader, to hold a vote on a bill to build the Keystone XL oil pipeline. She spent several days pleading with her Democratic colleagues to vote for it. On November 18, every Republican and 14 Democratic senators supported the bill. It fell short by a single vote. "FAIL MARY," declared a Politico headline.

Now, no one gives Landrieu much of a chance. "She's going to lose—it's just a matter of how much," Bernie Pinsonat, a pollster who works for both Republicans and Democrats, tells me. (Pinsonat began as a Democratic pollster, but that is no longer much of a viable occupation in this state.) Elliott Stonecipher, a Shreveport-based political analyst, adds: "She'll have trouble doing better than the 42 percent she got in the primary, and it could be worse than that." Many observers question Landrieu's campaign strategy, from her muddled message to the way she has allocated her funds. But, says Bob Mann, a former Democratic staffer who now writes a newspaper column and teaches at Louisiana State University, "She could be the best swimmer in the world, and it wouldn't matter. The tide is just too strong."

Still, Landrieu must go through the motions. She must play out the string. "I am fighting hard until the end," she announces in Hammond, surrounded by several local mayors and state legislators and a couple of dozen supporters. December in Louisiana: sunny and 80 degrees. Outside the canopy, a man and a woman with six platinum-blond children are waving a sign that says, "Babies are a blessing. Choose life." They are handing out pro-Cassidy literature. On the other side of the canopy, a woman is holding a Sierra Club sign that says, "Keep the Frack Out of My Water." (Landrieu is pro-fracking.)

Landrieu approaches a pair of grandmotherly women in Democratic Party T-shirts and puts her arm around one of them. They've been making phone calls on her behalf, and she wants to know if the word is getting out—are people starting to see that this race isn't about the national parties, but about the local matters Landrieu wants it to be about? Has anything changed since the primary?

"I haven't seen any change," says Jeanne Voorhees, a 72-year-old with a cross necklace. Bernadette Powell, who is 68 and moved to this part of the state when she lost her New Orleans house in Hurricane Katrina, tells Landrieu she thinks she spent too much time in this week's debate haranguing Cassidy about a late-breaking scandal involving his Louisiana State paycheck.

"Focus on your record! Your record is good!" Powell tells Landrieu.

"You would think people would be focused on that," Landrieu says, shaking her head, "but it's tough."

A couple of hours later and an hour down the road in Baton Rouge, a reporter asks Landrieu how she feels about her party abandoning her, and her defiant facade cracks a bit. "I am extremely disappointed in the Democratic Senatorial Committee," she says. "This is a fight worth fighting. I mean, I have a very good record! Records should matter!" It doesn't seem to occur to Landrieu that it is precisely her record—of supporting an unpopular president and voting for all his major initiatives, including the Affordable Care Act—that voters object to.

Landrieu's soft voice takes on a pleading cast as she contemplates the electorate that, having sent her to the Senate three times, now seems poised to cast her out. Why can't they appreciate all she's done for them? "People say they want somebody to break the gridlock," she says. "I've been breaking it up and busting it up for 18 years! I mean, what is not clear about this? They want politicians that are honest, I've been honest! I've served with integrity!"

For a moment, Landrieu seems like she might go off the rails completely. Then she regains her composure, thanks the press, and walks away. As she leaves, she tells a supporter she is going to have dinner with her son.

* * *

In what are likely the waning days of Landrieu's career, she has become an exotic specimen—the Last Southern Democrat. She is a political version of Martha, the last passenger pigeon, who died alone in the Cincinnati Zoo exactly 100 years ago. Like passenger pigeons, who were once so numerous they constituted a plague on the land, Democrats once dominated this region. Even after the civil-rights era and Richard Nixon's Southern Strategy began to realign the parties in the 1960s, individual Democrats at the state and federal level still won plenty of elections. But that has changed dramatically since Barack Obama was elected in 2008.

As recently as 2007, Democrats controlled both houses of the Louisiana legislature and seven of nine statewide offices, including the governorship. Today Republicans have large majorities in both houses, and Landrieu is the last statewide elected Democrat. As the Associated Press recently noted, if Landrieu loses, Democrats will not control a single governorship, Senate seat, or legislative chamber from the Carolinas to Texas.

Southern Democrats were once so loyal it was said they would vote for a yellow dog before they would vote for a Republican. In the 1990s, conservative, mostly Southern Democrats in Congress began calling themselves "blue dogs," because, one Texas Democrat joked, if you choke a yellow dog, it turns blue. But of course, if you choke any animal long enough, it will die.

"It appears that the Democratic Party in Louisiana, with the exception of the odd majority-minority district, is moribund for the foreseeable future," Pearson Cross, a political scientist at the University of Louisiana at Lafayette, tells me. The approximately one-third of the population that is black is overwhelmingly Democratic, but white voters have fled the party. Landrieu, who got 33 percent of the white vote in 2008, drew just 18 percent of whites in last month's primary.

Cassidy's campaign, like those of almost every Republican this year, has consisted almost entirely of putting Landrieu and Obama in the same sentence. A chyron hovers at the bottom of the screen throughout all of his television ads: "LANDRIEU + OBAMA = FAILED POLICIES." It worked for the others, and it seems to be working for him. Gone are the days when Democrats could convince voters to ignore their party label. "Obama is so unpopular that you can't be a Democrat and run as a Republican anymore," says Pinsonat, the pollster. "We've got people running for city council—it's funny to watch—and all of a sudden they find themselves in a television ad with their picture next to Barack Obama. Once that happens, you're dead. You've got no chance. That's why we don't have any white Democrats left in the House and Senate."

America's changing demographics have been touted by Democrats as a nearly unbeatable long-term coalition, particularly in presidential elections. In both his elections, Obama overwhelmingly won minorities and the young, groups whose share of the electorate is only set to grow, while Republicans' most loyal voters were the shrinking, dying cohort of the old and white. But the 2014 results have raised new doubts about this strategy. Many Democrats had seen the Obama tally as a sort of floor for the party going forward; without Obama on the ballot, they assumed they could continue to turn out minority voters while doing better with whites. But now it looks as though it may instead be a ceiling, as minorities stay home or even vote Republican while white voters turn on Democrats in increasing numbers.

"I don't think it's racial," Mike Foster, the Republican former two-term governor of Louisiana, now 84 and retired, tells me, without my having asked. I've reached Foster at his home outside Franklin, Louisiana, in a former sugar-plantation house known as Oaklawn Manor. "You know, the races have gotten along down here for years. Look at what's happening in Ferguson right now. We don't have a bunch of people running out in the street hollering about that."

Foster began his political career as a Democrat, but became a Republican during the course of his successful first run for governor in 1995. Even he now seems a bit surprised by the suddenness of the state's political transformation.

"I think what has changed it is that this is a hardworking state. People work hard, and they really don't take to people who are on the dole," he continued. "You'd better not be supporting people who are sitting on their front porch while I'm trying to work! You drive around these small communities, you see a lot of able-bodied people sitting around, when you know there's work to be had .... That's the only thing I can figure. This part of the country, people have been raised by families who worked very, very hard. But now we've got a president who loves to sit down every day and see how much he can give away of what they make."

* * *

Cassidy, Landrieu's opponent, is a lanky man with a barely tamed shock of fluffy white-blond hair and a remarkably awkward manner. Jumpy and seemingly uncomfortable in his own skin, he has a bony, plastic, Jim Carrey-like face that breaks at odd moments into a frighteningly intense grin. When the two candidates met for a debate at a television studio in Baton Rouge on Monday, he strode to his podium and shook Landrieu's hand, then stared in the other direction, the grin flashing on and off his face for the benefit of the cameras.

The debate was my one opportunity to see Cassidy this week. He held no other public events, and his campaign did not respond to requests for an interview. Shortly before a scheduled Wednesday evening rally with former presidential candidate Rick Santorum in Shreveport, Cassidy canceled his Wednesday and Thursday appearances in the state, citing the need to stay in Washington for House votes. He is transparently running out the clock, avoiding potential mistakes by avoiding campaigning altogether. With Landrieu barnstorming the state while Cassidy goes underground—but continues to dominate the airwaves—the campaign has an oddly lopsided feel.

Landrieu's campaign believes Cassidy is trying to avoid answering questions about the payroll issue—records obtained by a local blogger and published shortly before Thanksgiving show that he billed the university hospital for hours worked on days he was actually in Washington for committee hearings. The story does not appear to have originated with the Landrieu campaign, which dearly wishes it would have broken a few weeks or months earlier.

During the debate, Cassidy speed-talks for a few minutes, then realizes what he's doing and slows to a painfully deliberate pace. At one point, trying to summon something to mind, he squints his eyes shut, clenches his hands at his sides, tilts his chin skyward, and says, "Umm," like an elementary schooler straining to remember a multiplication table. He begins almost every answer with "Yeah," and in one particularly strange and patronizing move, he interrupts Landrieu when it is supposed to be her turn to ask a question. "Is this Doctor Double-Dip?" Cassidy says.

"Yes," Landrieu answers.

"Yeah, I looked at your notes," he says, smirking and swigging from his glass of Coke.

But Cassidy has the advantage of clarity. While Landrieu tries to triangulate on every issue—abortions are tragic, but women should make the decision; the Second Amendment is important, but certain people shouldn't have guns; the health-care bill is flawed, but it's helped many people—Cassidy's positions are easy to summarize. "Yeah, I have an A-plus from the NRA," he says. "Senator Landrieu has a D."

Even Cassidy's supporters tend to euphemistically describe him as "not a natural politician." (Cedric Richmond, a Democratic congressman from New Orleans, memorably told Politico, "Dude is weird.") Observers predict Cassidy will be a middle-of-the-road Republican senator who votes reliably with soon-to-be-Majority Leader Mitch McConnell and devotes himself to energy and health-care issues. That is, assuming he doesn't spend his time "fighting subpoenas," as Landrieu predicts.

The day after the debate, at Landrieu's rally in Baton Rouge, she is joined by John Breaux, a former Democratic senator who left office in 2005 and is now a Washington lobbyist. Landrieu, when I ask her about what happened to Democrats nationally on November 4, does not want to talk about it—"My focus has been here in Louisiana," she says. But Breaux interjects. "A lot of Republican candidates for Senate have been able to convince people of their state that if you don't like President Obama, you should somehow not vote for your own incumbent senator, even though they've done a great job for your state," he says. "And that's the case here."

To bring the party back, he believes Democrats need a new Bill Clinton who can reposition the party toward the center. The day after the election, Breaux tells me, he called Clinton to suggest restarting the centrist Democratic Leadership Council. "The perception now is that the Democratic Party is too far to the left and the Republican Party is too far for the right," he tells me. "The majority of Americans are somewhere in the middle."

Many Democratic partisans believe otherwise, and have been insisting since the election that the party needs to become more progressive, not less. Breaux disagrees. "That's a serious mistake," he says. "We didn't lose these elections because we weren't liberal enough. We lost these elections because we were perceived as being a party that was not a centrist party."