MANCHESTER, N.H. — He was once the presumptive Democratic nominee, but on Tuesday, Joe Biden fled New Hampshire with his tail between his legs — and headed to South Carolina, where he watched his dismal results come in.

Sensing a humiliating defeat in the Granite State, Biden’s team announced the former vice president was throwing in the towel before the polls had even closed, spending Tuesday evening at a campaign launch party in Columbia, SC, about 1,000 miles away.

Biden ended up placing fifth in New Hampshire, capturing less than 9 percent of the vote and none of the state’s pledged delegates.

Despite the potentially disastrous showing, Biden rallied with supporters in South Carolina, insisting the first two states in the primary season are not bellwethers.

“Tonight, though, we just heard from the first two of 50 states. Just two,” Biden said. “It ain’t over, man. We’re just getting started.”

Political experts, however, saw a campaign with little to offer new voters.

“It’s never a good sign when you leave a primary before the results are in,” said University of Southern California political scientist Robert Shrum, who managed Democrats Al Gore’s and John Kerry’s White House campaigns.

“It’s probably not a good message to people who might be inclined to vote for him, but on the other hand, [Biden’s team is] anticipating a mediocre result.”

Biden started out as the untouchable front-runner when he entered the presidential race in April 2019, but 10 months later, he is engaged in a battle for his political survival after a devastating fourth-place finish in the Iowa caucuses.

Political experts said the 77-year-old Biden’s campaign has been dealt repeated blows by poor debate performances, his family’s ties to the Ukraine scandal, cringe-worthy gaffes, low-energy events and a lackluster ground game in early states.

University of Iowa political science professor Timothy Hagle said that at least one of his colleagues was turned off by Biden because of the “whole Hunter thing” — referring to Hunter Biden’s controversial and lucrative stint on the board of the Ukrainian gas company Burisma while his then-veep dad was in charge of US policy on Kiev.

“A lot of Democrats, their main issue is they want someone who can beat Trump,” Hagle told The Post.

“They thought it was a weakness in the general election that Trump could go after,” the professor added, referring to the Hunter debacle, which was amplified during the House impeachment hearings and Senate acquittal of President Trump.

Shrum and Hagle suggested that Biden has focused too much on the past instead of promoting his vision for the future.

Poor debate performances also left doubt in voters’ minds about his strength as a candidate, they said.

“So much of his argument has been about the past, and elections really are about the future,” Shrum said.

“Al Gore had a line that elections are not a reward for past performance, and I think that’s largely true.”

Hagle said Biden also had a terrible ground game in New Hampshire and Iowa, where his team couldn’t decide how much to commit to the lily-white states.

“I’m not sure how hard he was working in New Hampshire. I got the impression he was working harder in Iowa … He was at least trying,” Hagle said.

“Now, to leave [New Hampshire] on night of election … it’s a tough thing for him. It doesn’t make his campaign look good,” he said.

Biden was already fighting off questions about his electability after the Iowa debacle when a Quinnipiac national poll released Monday showed that Sen. Bernie Sanders had pulled well ahead of him.

At his peak last May, Biden led Sanders by 26.8 percentage points, according to a RealClearPolitics national average. By Monday, Sanders was leading him 25 percent to 17 percent.

By ditching New Hampshire, Biden has essentially bet the house on the Feb. 29 primary in South Carolina, where he has by far the most support from black voters.

He is hoping for a resounding victory there, in what his campaign is betting will be a firewall state for him.

But even that scenario seems to be in jeopardy, the experts said.

The Quinnipiac poll showing Sanders beating him also has nearly half of Biden’s previous African American supporters abandoning him nationally — with many now backing Mike Bloomberg.

“Biden needs to recover and recover in a very strong way in South Carolina,” Shrum said.

“To do that, he would need to defy history. People say Bill Clinton lost the first several primaries in 1992, but his second-place finish in New Hampshire was effectively victory.”

Added Hagle, “Maybe Biden ends up being right and South Carolina ends up being a big victory for him, but whether that is enough to save his campaign, we just don’t know.”

Both pundits suggested that Biden’s exit would be an opening for Bloomberg and Sen. Amy Klobuchar of Minnesota, fellow moderates.

Bloomberg’s “bet was Biden would falter,” Shrum said. “That was the whole premise of his campaign — and so far, it looks like a pretty smart premise.”