There remains the formality of a runoff election on Saturday — but as far as the national Democratic Party is concerned, three-term Sen. Mary Landrieu of Louisiana is presumed dead.

The Democratic Senatorial Campaign Committee canceled its plans to buy ads and is not lifting a finger — or writing a check — to save her. Nor are any of the heavy-hitting Democratic outside groups.

“I am extremely disappointed,” she said Tuesday of the DSCC’s decision. “You know, they just walked away from this race.”

So with the odds stacked heavily against her, Landrieu soldiers on virtually alone — this year’s political equivalent of those holdout Japanese infantrymen who were discovered waging war on remote Pacific islands decades after World War II had ended.

“The political true bloods have moved on to 2016,” said Brad Dayspring, a spokesman for the National Republican Senatorial Committee.

It wasn’t supposed to be this way. There was a time when both parties thought control of the Senate might hinge on the results of the Bayou State runoff election.

But whatever happens Saturday will not change the balance of power on Capitol Hill. By comfortably winning a majority on Election Day, Republicans turned the contest between Landrieu and her challenger, Republican congressman Bill Cassidy, from a campaign firefight into a footnote.

And whether she wins or loses, her Energy Committee chairmanship — the gavel she touted as Exhibit A of her indispensability to this oil-producing state — will be handed to the Republicans in January.

Emphasizing her clout as a chairman “was a short-sighted strategy, and when it was removed, they were left with nothing,” Dayspring said.

Her expected defeat would mark the end of an era. Landrieu — whose father is a former mayor of New Orleans and whose brother is the current one — is the last statewide Democratic elected official in Louisiana, a state her party once was presumed to own in perpetuity. If she loses, there will be no Democratic senator from the Deep South in the new Congress.

Landrieu’s home-state supporters are doing their best to gin up enthusiasm.

“It’s the fourth quarter!” bellowed Louis Reine, a feisty union leader rallying support for Landrieu as dusk settled over this city Tuesday evening. “We’re on the 10-yard line! We have a great quarterback! Will you call in the reserves? Will you get the team on the field?”

Landrieu dropped her right arm back and brought it forward as if to throw a football down the field. Supporters assembled steps from City Hall that crisp evening cheered her on.

But national Democrats speak — not for attribution, of course— as though they have already left the field and headed for the locker room. Their dispirited donors, they say, are tapped out and unwilling to open their wallets for a lost cause.

All along, Landrieu’s biggest hurdle has been the political climate, which helped the Republicans take eight other ­Democratic-held Senate seats.

Party strategists also fault her campaign for disregarding advice from Washington and spending virtually all of its war chest on the November election, in hopes of avoiding a runoff. She got just over 42 percent of the vote in the eight-candidate field — eight percentage points shy of the showing she needed to win outright.

Now Cassidy is expected to consolidate most of the support of those who voted for the other six candidates.

Turnout could also be a problem for the Democrat. African American voters supported Landrieu overwhelmingly in the Nov. 4 all-party primary. But early-voting tallies suggest that the electorate will be whiter and more conservative in the runoff.

The DSCC canceled its reservations for ad time in Louisiana shortly after the Nov. 4 sweep. Senate Majority PAC, which spent millions trying to save Democrats, has not run a single spot in the runoff. Landrieu’s only reinforcements have come from some modest ad buys by a handful of groups, and some individual fundraising efforts, including one in New York on Monday headlined by former secretary of state Hillary Rodham Clinton.

Though Republicans have also scaled back their efforts in Louisiana — in their case, a sign of confidence — Landrieu is still being heavily outspent.

Pro-Cassidy groups are on pace to air more than $5.65 million worth of ads in the runoff, including more than $1.4 million from the National Republican Senatorial Committee, records show.

An analysis of data by Kantar Media/CMAG for the Center for Public Integrity found that outside groups allied with Cassidy have put up about 6,000 ads during the runoff period. That compares with fewer than 100 by those supporting Landrieu, whose most active supporter has been the Humane Society Legislative Fund.

The lack of a national rallying cry has driven Landrieu to frame the race as a local contest, highlighting her record on jobs, education and disaster relief.

At a sun-soaked rally across the Mississippi River from New Orleans on Tuesday morning, with the sound of pile drivers echoing in the background, Landrieu’s standard stump speech — delivered partly in the past tense — seemed to sound an almost elegiac note. “It has been my joy to represent you, to fight for you and to win for you in Washington,” she said.

“When we heard that the national Democratic Party pulled out, it really kind of stunned us,” said Harriet Jones of Baton Rouge, who backs Landrieu. “When we thought about it, we thought, you know, they’ve just given up on us. It’s disappointing.”

On her own, Landrieu has flailed, veering from one issue to another. Last month, she spearheaded a push to pass a measure pressuring President Obama to construct the Keystone XL pipeline. It fell short in the Senate by one vote thanks to her own party’s opposition. But it passed in the House, where Cassidy’s name was strategically attached to it.

More recently, Landrieu has been hammering Cassidy over his work as a part-time physician and teacher at Louisiana State University. After a local blog revealed details about Cassidy’s time sheets that raised questions about the amount of time he put in and whether it overlapped with congressional work, LSU said it would look into the matter. Cassidy, who has kept a lower profile than Landrieu in the final week of the race, has denied wrongdoing.

In contrast with Landrieu, he has leaned heavily on a less muddy — and more potent — line of attack: He has tied Landrieu to Obama, using the same argument that helped propel Republicans to victory across the country.

“Senator Landrieu represents Barack Obama. I represent you,” he said Monday at the candidates’ final debate.

According to Nov. 4 exit polling, Obama’s job approval rating in Louisiana was just 39 percent. Fifty-nine percent of voters disapproved.

In response, Landrieu has tried to strike a tricky balance between distancing her candidacy from the president and defending him.

“This isn’t about whether you like Bill Clinton as president or George Bush as president or Barack Obama as president,” she told reporters.

Yet Landrieu has also run a radio ad geared at African American voters who support Obama that suggests Republicans would try to impeach the president if Cassidy wins — an idea GOP leaders have not embraced.

This week, Landrieu accused Cassidy of being “disrespectful” to Obama. Asked what she meant by that, Landrieu said: “He refers to him by his last name. Constantly.”

She added: “If you are going to refer to the president of the United States, he’s at least earned the title that the people gave him when they elected him.”

If an unpopular president has brought her to this lonely moment, Landrieu is making a long-shot bet that he might help pull her through it.