The Republican House speaker, Paul Ryan, will not run for re-election, he announced on Wednesday amid Republican concerns over keeping their majority in the chamber at the midterms in November.

Speaking at a press conference on Wednesday morning, Ryan insisted that he had never sought the job of speaker and that his plans to retire were focused on his desire to spend more time with his children.

“If I’m here for one more term my kids will have only known me as a weekend dad,” said Ryan, whose plans have been the source of much speculation.

An aide said the congressman would serve his full term and retire in January.

“This is a titanic, tectonic shift,” an unnamed Republican told Axios, which first reported the news. “This is going to make every Republican donor believe the House can’t be held.”

Ryan was first elected to Congress in 1998. He was elected House speaker in 2015 after the former House speaker John Boehner retired. He was former GOP presidential nominee Mitt Romney’s running mate in 2012.

Friends said Ryan was ready to step down after passing a tax reform bill, one of his longtime goals, Axios reported.

On Twitter, Donald Trump, who has long had a contentious relationship with Ryan, praised him. Trump wrote: “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!”

Ryan initially declined to endorse Trump after he clinched the Republican nomination in the 2016 presidential election and said Trump’s attacks on a Hispanic judge were the “textbook definition of a racist comment”. But he later embraced Trump’s candidacy and has been an ally of the administration on Capitol Hill.

Ryan’s departure increases the number of House Republicans not seeking re-election in 2018 to 46. This record exodus has left many Democrats optimistic about their chances of picking up the 24 seats they would need to retake the majority.

In a statement, Tyler Law, a spokesman for the Democratic Congressional Campaign Committee, said: “Speaker Ryan sees what is coming in November, and is calling it quits rather than standing behind a House Republican agenda to increase healthcare costs for middle-class families while slashing social security and Medicare to pay for his handouts to the richest and largest corporations.”

The outgoing speaker dismissed the political impact of his decision on Wednesday. “I really do not believe that whether I stay or go in 2019 is going to effect a person’s individual race for Congress,” said the Wisconsin Republican.

Ryan’s retirement also opens up a potentially contentious leadership race to replace him. He was elected speaker in 2015 after his predecessor John Boehner was forced out by conservatives in the hard-right Freedom Caucus. Boehner’s presumptive successor, the House majority leader, Kevin McCarthy, was pushed out of the race by conservative pressure as well and Ryan took the gavel reluctantly as a compromise candidate.

The same pressures are likely to exist in a race to succeed Ryan with McCarthy and the minority whip, Steve Scalise, as the most likely contenders. However, any leadership election would take place after the midterms when Republicans would potentially no longer hold the majority.

The Wisconsin congressman also may leave behind a competitive race in his district. Although Trump won there by 10% in 2016, Barack Obama won the district in 2008 and narrowly lost it in 2012.

There is a competitive Democratic primary there between ironworker Randy Bryce and schoolteacher Cathy Myers. The only other Republican candidate running, Paul Nehlen, is a vocal antisemite and white nationalist.