Democratic Senator Joe Manchin said on Thursday that he would not run for re-election in 2024, signalling the potential for political upheaval in the narrowly divided Senate.
“After months of deliberation and long conversation with my family, I believe in my heart of hearts that I've accomplished what I set out to do for West Virginia,” he said in a video posted on X, the platform formerly known as Twitter.
“I've made one of the toughest decisions of my life and decided that I will not be running for re-election to the United States Senate.”
Mr Manchin has served as senator for West Virginia since 2010 over the course of two terms. Prior to entering the chamber, he was the state's governor for five years and served as secretary of state before that.
"For more than 40 years - as a state legislator, a secretary of state, a governor and a senator - Joe Manchin has dedicated himself to serving the people of his beloved West Virginia," President Joe Biden said in a statement.
"During my time as vice president and now as President, Joe and I have worked together to get things done for hardworking families."
Mr Manchin's departure could lead to a major political shift in the Senate.
The chamber is currently controlled by the Democrats, who holds 48 seats; another three seats are held by independent politicians who tend to vote with the Democrats. The Republicans hold 49 seats.
Despite helping to give Democrats control of the Senate, Mr Manchin has long posed a challenge for the party. He has held a moderate and sometimes conservative stances on many hot-button issues, forcing Democrats to make concessions to pass legislation.
In West Virginia, Republicans control the governor's office and other congressional seats, and Donald Trump won 69 per cent of the vote over Democrat Mr Biden in the 2020 election.
“We like our odds in West Virginia,” Steve Daines, chairman of the National Republican Senatorial Committee, said in a statement following Mr Manchin's announcement.
Mr Manchin was not clear about what might come next for him.
“What I will be doing is travelling the country and speaking out to see if there's an interest in creating a movement to mobilise the middle,” he said in his video.