House speaker Paul Ryan has announced he will leave Congress at the end of his term next year.
The Republican lawmaker told reporters of his plans to retire on Wednesday morning, following a closed meeting with members of his party.
He will serve out his term until January, when he will not stand for re-election.
Presenting the decision as a personal matter, the Wisconsin representative said he did not want to be a "weekend dad" as his children grew up.
"I have given this job everything I have," he said. "We're going to have a great record to run on."
Shortly after the official announcement Donald Trump expressed his support for Mr Ryan.
"Speaker Paul Ryan is a truly good man, and while he will not be seeking re-election, he will leave a legacy of achievement that nobody can question," the President tweeted. "We are with you Paul!"
Speaker Paul Ryan is a truly good man, and while he will not be seeking re-election, he will leave a legacy of achievement that nobody can question. We are with you Paul!
— Donald J. Trump (@realDonaldTrump) 11 April 2018
The public announcement was preceded by a closed door meeting with Republican representatives, in which Mr Ryan delivered his news in a sombre tone, reading directly from prepared notes.
Brendan Buck, an adviser to Mr Ryan, said in a statement: "After nearly 20 years in the House, the speaker is proud of all that has been accomplished and is ready to devote more of his time to being a husband and a father.
"While he did not seek the position, he told his colleagues that serving as speaker has been the professional honour of his life, and he thanked them for the trust they placed in him."
At a time of insecurity regarding their majority in the House of Representatives, the news will likely prompt a scramble among Republicans for the key post.
Key figures including Kevin McCarthy and Steve Scalise declined to comment on whether they would seek to succeed Mr Ryan when quizzed by journalists.
"The only thing I'm focused on is doing whatever we can to keep the majority," Mr McCarthy, who withdrew a bid to become speaker before Mr Ryan took the job, said.
Mr Ryan served more than 20 years in Congress, and while he appeared to share little in common with Mr Trump, he went on to play a central role in the current US administration.
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