US Democrats' chances of holding onto their majority in the Senate in 2024 were thrown into doubt Thursday when the party's powerful lawmaker Joe Manchin announced he would not run for reelection in conservative West Virginia.

The departure of Manchin, a centrist Democrat whose often-contrarian voting record has frequently thwarted President Joe Biden's legislative agenda, presents Republicans a clear opportunity to pick up his seat in the narrowly divided chamber in next year's election.

"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," Manchin said in a video posted on his account on X, formerly known as Twitter.

Manchin referenced his independent streak in explaining his resignation.

"Every incentive in Washington is designed to make our politics extreme. The growing divide between Democrats and Republicans is paralyzing Congress and worsening out nation's problems," he said.

"The majority of Americans are just plain worn out."

With Manchin gone, Democrats would have to hold every one of their seats and pick up another to maintain their current 51-49 Senate majority in 2024 -- or else hold every seat and keep the White House, to allow Vice President Kamala Harris to act as a tie-breaker.

Electing another Democratic senator would be a tough task in West Virginia, where nearly 69 percent of voters picked Donald Trump over Biden in the 2020 presidential race.

Democrats' uphill 2024 Senate battle will also come as the party tries to overtake the Republicans' thin majority in the House of Representatives.

Manchin, who has served in the Senate since 2010, played a key role in passing Biden's signature Inflation Reduction Act climate law last year -- though the West Virginian had first used his influence to tank the president's original and larger Build Back Better plan.

Despite that tense history, Biden in a statement hailed the "real progress" Manchin achieved in his career as the two men "worked together to get things done for hardworking families."

The announcement comes after Democrats notched solid wins in state elections Tuesday, even as Biden faces grim poll numbers against his likely Republican opponent Trump one year out from the presidential vote.

Speculation swirled over whether Manchin's future could include a third-party shot at the presidency, as the 76-year-old said, "public service has and continues to drive me every day."

"What I will be doing is traveling the country and speaking out to see if there's an interest in creating a movement to mobilize the middle and bring Americans together," he said.

"We need to take back America and not let this divisive hatred further pull us apart."