Sen. Joe Manchin leaves Democrats guessing: Will he run for reelection ... or the White House?

Many Democrats are starting their least favorite sexennial waiting game: hoping Sen. Joe Manchin runs for reelection while having little recourse to sway the willful West Virginian.

Manchin, one of the most conservative members of his party and an occasional thorn in the side of Senate leadership and the Biden White House, is keeping his cards close to the vest, as he often does before his seat is up, multiple sources told ABC News.

He has said he will not announce until December his future plans -- which he's indicated could include running for reelection, retiring from politics or waging a bid for the White House in 2024.

All three options have vastly different ramifications. (Manchin's office did not comment for this story.)

A reelection bid would put in play for Democrats a Senate seat that observers believe only Manchin could realistically win for them, given his track record and how West Virginia has increasingly favored Republicans. Manchin retiring would likely forfeit his seat to the GOP, because no other state Democrats have a similar stature. And a presidential bid would not only seriously dim Democrats' hopes to hold the Senate seat -- it could also create some friction in the White House race.

Manchin's recent support of GOP legislative efforts and opposition to Biden administration nominees may suggest he's planning on running -- but those in his party can't say for what, according to the sources who spoke with ABC News. Interviews with more than a dozen Democrats in West Virginia and Washington suggest there are few others, outside Manchin's inner circle, who can influence him and he famously chafes at outside pressure.

"If you're talking about in terms of the waiting game with the D.C. crowd, whether it's Republican opponents or the Democratic Party, whatever you might want to call it, no. Not one iota do I think he's going to factor that in," said one former Manchin aide, who was not authorized to speak publicly.

"I know he's very sincere when he says he's gonna wait to the end of the year," the ex-staffer said. "And I don't think there's anything that's gonna lead him to change his mind on that."

Manchin was a pivotal vote in the 50-50 Senate in the last Congress but has made a point of reasserting independence from the White House and national Democrats since the midterms, which saw Democrats gain another Senate seat that lessened Manchin's power and the pressure on him.

He threatened last week to block some of President Joe Biden's nominees from his perch as chair of the Senate Energy and Natural Resources Committee and to determine if potential appointees are "political partisans first or Americans first" over frustration with the administration's implementation of the Inflation Reduction Act.

He's also said he identifies as "an American" when asked if he's a Democrat and has thrown his support behind GOP efforts to overturn crime reforms in Washington, D.C., and a proposed Biden administration rule on water regulations.

Manchin's recent moves could be crucial to running as a Democrat in West Virginia or challenging a Democratic president, observers said. But those who know him warn against trying to decipher any hidden messages.

"He doesn't think about being in a campaign cycle or politics until he absolutely has to," said a second former aide who requested anonymity. "He's brutally honest and answers every question that you ask of him. … So, if he says he's thinking about it [running for reelection or the White House], he's thinking about it."

That hasn't stopped the guessing game.

"Everyone would like to know his answer yesterday but needs to get to a Zen place of acceptance because he is going to decide on his own timeline, and no one can really do anything about it," said Democratic strategist Eddie Vale.

Should Manchin run for reelection, he could face Republican West Virginia Rep. Alex Mooney, state Attorney General Patrick Morrisey, who lost to Manchin in 2018, or Republican Gov. Jim Justice, a former Democrat.

All three would likely give Manchin a stiff challenge given West Virginia's deep red hue, though Democrats in the state said Justice would be the toughest to beat.

"No matter what others do, Sen. Manchin will have a tough reelection. I think that's pretty well known by everybody. But he will not take anything for granted. He will work it hard, he knows the people of West Virginia, is a West Virginian and has dedicated his entire career to our state," said former Rep. Nick Rahall, D-W.Va., a surrogate on Manchin's last reelection bid.

Rahall estimated that a run next year would be "quadruply" more difficult than in 2018, when Manchin won by about 3 points.

Still, virtually every person interviewed said Manchin is the only one who stands a chance in the state -- and that his decision holds outsized sway beyond just his seat.

"There is no Democrat who can ever win that Senate seat again," the second former aide said, adding, "If you want to stay in the majority, you need his seat."

A run for president, meanwhile, would likely be on a third-party ticket, and interviews indicated little consensus over how that could play out next year, though Manchin may pull from moderates in both parties.

"Does it hurt the Republicans or does it hurt the Democrats? I don't know the answer to that," said Mark Updegrove, ABC News' presidential historian.

But "will Manchin be a fly in the ointment? Without question," Updegrove said.

While Manchin makes up his mind, his colleagues are approaching his future gingerly, sources said.

"I really don't think that the caucus as a whole is making a concerted effort to push him on timeline," said one Senate Democratic aide. "I think there are a lot of voices in the Senate that think he is a valuable partner on bipartisan legislation and would like to see him run for reelection."

Manchin is partly able to take his time making a decision because of the lack of a Democratic bench in West Virginia, with others needing time to set up a campaign should he pass on reelection, though he also could be biding his time to make the most out of his current term.

"The worst thing in the world he could do for his own power and influence on behalf of West Virginians would be to turn himself into a lame duck by announcing prematurely that he's not running," said one Democratic strategist who works on Senate campaigns. "And the worst thing he could do if he is running for reelection is announce prematurely and … stop any possibility any Republican would want to work with him."

The National Republican Senatorial Committee has already released an ad hammering Manchin over his vote for a COVID-19-era rescue package and warned that it intends to "hold Joe Manchin accountable."

However, should Manchin decide to wage a Senate or presidential campaign, allies warn against underestimations by either Republicans or Democrats.

Both former aides who spoke with ABC News touted Manchin -- whose family coal company made him wealthy and still maintains a prominent footprint in his state -- as a fierce retail politician, a tactic they say helps him transcend his state's partisan fray.

"When he interacts with everyday Americans, at that moment, he's as real as he is when he interacts with people [who are] so-called powerful. That's a powerful quality in an era where people are sick and tired of traditional politics," the first former aide said.

"The politics of any campaign are going to make it a challenge. There's no question about that," this person added. "But if anyone thinks if he decides to run for Senate that it's gonna be an easy race -- or if he decides to run for president, they think it's going to be an easy race -- OK, keep thinking that. It may not serve you very well in the long run, but keep thinking it."