The move fuels speculation he could mount a third-party presidential bid.
Sen. Joe Manchin, a West Virginia Democrat, announced Thursday that he will not run for reelection next year, fueling speculation over whether he plans on mounting a third-party White House bid and immediately complicating his party's chances of holding the Senate past 2025.
"After months of deliberation and long conversations with my family, I believe in my heart of hearts that I have accomplished what I set out to do for West Virginia," Manchin, who had been at the center of a monthslong waiting game in Washington as he mulled his future, said in a statement.
Confirming his exit from the Senate is likely to make it much more difficult for Democrats to hold his seat, given that West Virginia largely elects Republicans.
As one of the most conservative members of his party, and a former governor of the state, Manchin repeatedly fended off GOP challengers including in 2018, when he won by less than 20,000 votes.
Democrats currently hold the Senate 51-49 and will be defending seats in several red states in next year's elections, including in Montana, Ohio and West Virginia.
West Virginia's Republican governor, Jim Justice, who is term-limited, has already announced plans to run for Manchin's seat. Justice will face West Virginia Rep. Alex Mooney in the primary.
In a statement following Manchin's announcement, Montana Sen. Steve Daines, who leads Senate Republicans' campaign arm, issued a brief statement: "We like our odds in West Virginia."
Manchin insisted on Thursday that he will remain involved in politics. He seemingly alluded to the possibility that he could mount an outside presidential bid amid what polls show is widespread dissatisfaction with both President Joe Biden and former President Donald Trump, currently the likely Democratic and Republican nominees next year.
"I have made one of the toughest decisions of my life and decided that I will not be running for reelection to the United States Senate, but what I will be doing is traveling the country and speaking out to see if there is an interest in creating a movement to mobilize the middle and bring Americans together," Manchin said.
Multiple sources told ABC News that he's not ruling out a White House run.
"Nothing is off the table," one source said, adding, "No specific decisions have been made other than a commitment to find a way to change the country's political dialogue."
Manchin's office is not commenting, referring reporters to his video statement.
Having served in the Senate since 2010, Manchin's moderate positions have at times been a thorn in the side of his increasingly left-leaning party. He's used Democrats' slim majority in the Senate to secure major concessions from the Biden administration on some of its legislative accomplishments, including government spending.
He also emerged as an influential player in the passage of Biden's signature Inflation Reduction Act, providing one of the final votes needed for the Democratic-only bill that invested billions in deficit reduction, invested in U.S. renewable and non-renewable energy production, allowed Medicare to negotiate on some prescription drug prices and capped the cost of insulin for seniors at $35.
Amid the mounting speculation about his future -- whether he would seek another term or leave the Senate for something else -- Manchin previously told reporters that he would make a decision in December.
He has repeatedly expressed frustration with the country's two-party system, at times insisting that unless it changes it would be the "downfall of our country."
Over the summer, he spoke at a forum in the early voting state of New Hampshire that was hosted by No Labels, the group considering offering a third-party "unity" ticket in 2024. In their own statement, No Labels called Manchin "a tireless voice for America's commonsense majority and a longtime ally."
"The Senate will lose a great leader when he leaves," the group said.
No Labels acknowledged its own plans for 2024, which it said have still not been determined: "We are gathering input from our members across the country to understand the kind of leaders they would like to see in the White House. As we have said from the beginning, we will make a decision by early 2024 about whether we will nominate a Unity presidential ticket and who will be on it."
In his statement on Thursday, Manchin lamented the division in Washington and again signaled the need for some sort of consensus candidate.
"Every incentive in Washington is designed to make our politics extreme. The growing divide between Democrats and Republicans is paralyzing Congress and worsening our nation's problems. The majority of Americans are just plain worn out," Manchin said.
"I know our country isn't as divided as Washington wants us to believe," he said. "We share common values of family, freedom, democracy, dignity and a belief that together we can overcome any challenge. We need to take back America and not let this divisive hatred further pull us apart."