Senator Joe Manchin III, the conservative West Virginia Democrat, on Friday announced that he would not seek the White House in 2024, ending months of speculation that he might challenge President Biden as an independent candidate.

“I will not be seeking a third-party run,” he said in a speech in Morgantown, W.Va. “I will not be involved in a presidential run.”

Since Mr. Manchin, 76, announced in November that he would not run for re-election, he had been the subject of months of public and private guesswork about whether he would seek the presidency. In particular, he had flirted with becoming the candidate for No Labels, a centrist group aiming to recruit a third option against Mr. Biden and former President Donald J. Trump.

But on Friday, he pledged, “I will not be a deal breaker or a spoiler.”

Mr. Manchin’s decision takes off the table the highest-profile candidate the leaders of No Labels had sought to attract. They had told donors and allies in recent months that they planned to name a Republican to lead their ticket, ostensibly taking Mr. Manchin out of contention, but they have so far not found any takers.

Former Gov. Larry Hogan of Maryland, a moderate Republican, quit the No Labels board in January. After endorsing former Gov. Nikki Haley of South Carolina for president, he announced his own campaign for Senate in Maryland last week.

Since Mr. Manchin said he would not run again, Senator Chuck Schumer of New York, the majority leader, has been pleading with him to formally leave the Democratic Party and run as an independent, according to two people familiar with their conversations.

West Virginia’s deadline to file to run in a Senate primary race was in January, but independent candidates have until Aug. 1 to declare their candidacy.

Without Mr. Manchin, Democrats have written off the possibility of holding the seat in deep-red West Virginia. They have an uphill battle to try to keep control of the Senate, with several incumbents defending seats in states won by Mr. Trump.

Mr. Manchin has been known in the Senate for bipartisan deal-making and also for frustrating some of his party’s most ambitious policy goals.

During his speech on Friday at West Virginia University, Mr. Manchin denounced the state of Congress, which he described as the most dysfunctional body he had ever experienced.

“This will be the least productive, most destructive Congress that we have ever had in the history of the United States,” he said.

Carl Hulse contributed reporting.