US House Speaker Paul Ryan has announced he will not run for re-election and will retire next year, injecting another layer of uncertainty as Republicans face worries over losing their majority in the autumn.
The Wisconsin Republican cast the decision as a personal one, saying he did not want his children growing up with a "weekend dad".
He told reporters he believes he is leaving with strong accomplishments his party can sell to voters ahead of November elections.
"I have given this job everything I have," he said. "We're going to have a great record to run on."
Mr Ryan's plans have been the source of much speculation and will set off a scramble among his lieutenants to take the helm.
A self-styled budget guru, Mr Ryan had made tax cuts a centrepiece of his legislative agenda and a personal cause, and Congress delivered on that late last year.
House Speaker Paul Ryan: “I have given this job everything that I have… Today, I am announcing that this year will be my last one as a member of the House.” https://t.co/xQwme4fNHc https://t.co/RptA5tS9x0— CNN (@CNN) April 11, 2018
Mr Ryan, 48, first announced his plans at a closed-door meeting of House Republicans this morning.
His tone was sombre, and he read directly from prepared remarks.
Representative Mark Walker said an emotional Mr Ryan "choked up a few times trying to get through" his remarks to colleagues and received three standing ovations.
Moments later, Mr Ryan told reporters that if he were to stay for one more term, his children - now all teenagers - would only know him as a weekend father.
"I can't let that happen," he said.
The Speaker called extended family and a few close friends on Tuesday night and alerted a few staff.
Today, he called the president and the vice president and informed the rest of his staff before going to the conference meeting, officials said.
Mr Ryan, who has had a difficult relationship with President Donald Trump, thanked the president for giving him the chance to move the Republican party ahead.
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) April 11, 2018
Mr Ryan, a Republican from Janesville, Wisconsin, was first elected to Congress in 1998.
Along with Representatives Eric Cantor and Kevin McCarthy, he branded himself a rising "young gun" in an ageing party.
He became presidential nominee Mitt Romney's running mate in 2012.
Mr Ryan was pulled into the leadership job by the abrupt retirement of House Speaker John Boehner in 2015.
Mr Boehner had struggled to wrangle the chamber's restless conservative wing and failed to seal the big-picture deals on fiscal policy he sought.
Mr Ryan had more trust with the hardliners in the House but had no more success in brokering fundamental reform of entitlement he sought.
He ultimately had to wrestle with another unexpected challenge: Mr Trump, a president with little of Mr Ryan's interest in policy detail or ideological purity.
The two have had not had a close working relationship.
House Majority Leader McCarthy, a Republican from California known to be closer to Mr Trump, is expected to seek the Speaker post.
He is likely to compete with House Majority Whip Steve Scalise, of Louisiana, for the job.
Both men spoke at the closed-door meeting, delivering tributes to Mr Ryan.
In Wisconsin, the most likely Republican candidate is state Assembly Speaker Robin Vos, multiple Republicans in the state said.
Another Republican mentioned as a potential candidate is longtime Ryan family friend and Ryan backer Bryan Steil, a lawyer and member of the University of Wisconsin Board of Regents.
BREAKING: WE JUST REPEALED PAUL RYANApril 11, 2018
Democrat Randy Bryce, a colourful ironworker who has cultivated an IronStache moniker, had been Mr Ryan's best-known challenger, drawing liberal support from around the country.
He had nearly $2.3m in the bank at the end of the first quarter.
Janesville teacher Cathy Myers was also running on the Democratic side.
The only declared Republican was Paul Nehlen, who was banned from Twitter for a series of posts criticised as racist or anti-Semitic.
- Digital Desk and Press Association