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President Biden already faces increasing doubts about his ability to win re-election next year, but now Democrats must confront the prospect of a growing list of potential 2024 challengers running as third-party candidates.

Major Democratic Party victories on Tuesday in the 2023 off-year elections gave Biden a much-needed boost after a slew of well publicized polls suggested he was trailing former President Donald Trump — the commanding GOP nomination front-runner — in a 2024 elections rematch.

But Thursday's announcement by 2016 Green Party presidential nominee Jill Stein that she will make another White House run next year was not welcome news for Team Biden.

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Jill Stein speaking into several microphones

2016 Green Party presidential nominee Jill Stein speaks at a news conference on Fifth Avenue across the street from Trump Tower in New York City on Dec. 5, 2016. (Drew Angerer/Getty Images)

Plenty of Democrats still blame Stein's 2016 campaign for putting Trump in the White House. Her vote totals seven years ago in the key battlegrounds of Michigan, Pennsylvania and Wisconsin topped Trump's margins over Democratic nominee Hillary Clinton in each state.

A couple of hours after Stein's news broke, moderate Democratic Sen. Joe Manchin announced he wouldn't seek re-election next year in heavily red West Virginia, deflating his party's hopes of holding their Senate majority in 2024.

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Manchin, who has openly flirted with the possibility of launching a third-party presidential bid, highlighted in his announcement video that in the coming months he would travel across the nation to see "if there is an interest in creating a movement to mobilize the middle and bring Americans together."

Joe Manchin headlines New Hampshire No Labels event

Democratic Sen. Joe Manchin of West Virginia, left, was co-headliner alongside former Republican Gov. Jon Huntsman of Utah at the 'Common Sense' Town Hall, an event sponsored by the bipartisan group No Labels, at St. Anselm College in Manchester, New Hampshire, on July 17, 2023. (John Tully for The Washington Post via Getty Images)

There has been rampant speculation Manchin could join a potential bipartisan national ticket that the centrist group No Labels is considering launching next spring.

Veteran New Hampshire-based political scientist Wayne Lesperance, the president of New England College, noted that Manchin potentially "creates new troubles for Team Biden’s re-election."

"Manchin has options — none of which are good for the president. He can decide to make a White House run on his own. He can join a No Labels-fueled effort to run. Even if he decided to just travel to battleground states and urge voters to support moderate or centrist campaigns, the impact will be felt by Team Biden," Lesperance said.

Biden is already facing independent presidential runs by environmental advocate and high-profile vaccine skeptic Robert F. Kennedy, Jr., a scion of the Kennedy family political dynasty, and outspoken progressive university scholar Cornel West.

Recent polls indicate that Biden faces rising concerns from American voters over his age. Surveys suggest that many Americans, including plenty of Democrats, do not want the president to seek a second term in the White House, and a handful of influential Democrats have suggested that the 80-year-old president should drop out of the 2024 race and pass the baton to a new generation.

The president is currently facing long-shot primary challenges from a pair of Democratic rivals.

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Three-term Democratic Rep. Dean Phillips of Minnesota, who launched a primary challenge against the president late last month, has been arguing that Biden cannot beat Trump in 2024. The recent polls released over the past week gave Phillips plenty of fresh ammunition.

Spiritual adviser and best-selling author Marianne Williamson, who is making her second straight White House run, is also challenging Biden.

President Biden speaks to United Auto Workers

President Biden speaks to United Auto Workers at the Community Building Complex of Boone County in Belvidere, Illinois, on Thursday. (AP Photo/Paul Beaty)

Pointing to the strong performance by Democrats at the ballot box earlier this week, Biden 2024 re-election campaign manager Julie Chavez Rodriguez said Thursday that "we’ve heard the press and pundits count Joe Biden out time and time again, but we know he always proves them wrong."

"On Tuesday, voters in states across the country proved the pundits wrong," Chavez Rodriguez emphasized.

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During a trip Thursday to speak to the United Auto Workers in Illinois, Biden himself took aim at media coverage of the latest surveys suggesting he's losing to Trump in hypothetical 2024 matchups.

"Because you don’t read the polls," Biden pointed out. "Ten polls. Eight of them, I’m beating [Trump] in those places. Eight of them. You guys only do two. CNN and New York Times. Check it out. Check it out."

Asked if he believed he was trailing Trump in the key battlegrounds, the president answered, "No, I don’t."

It's not clear if Manchin will join a potential national ticket that No Labels is considering launching, and independent candidates like Kennedy and West face high hurdles when it comes to getting on the presidential ballot next year in states across the country.

In addition, some of these candidates could theoretically pull more support from Trump than Biden in a likely multiple-candidate 2024 presidential election field.

Lesperance noted "the fact that polling numbers suggest that voters are unhappy with the prospect of another Trump-Biden contest, add that to the candidacies of Jill Stein, RFK Jr., and Cornell West, and it’s hard not to conclude that the president’s prospects are more difficult. Political headaches abound." 

Biden — at least outwardly — isn't fazed. 

On Thursday, at a fundraising event in Chicago, he pointed to the election results from earlier in the week and said, "Democrats had an incredible night once again."

Referring to Trump, the president asserted that "we haven’t stopped winning, and he hasn’t stopped losing."

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.