US House of Representatives speaker Paul Ryan has endorsed Donald Trump's bid to be US president, bringing an end to the extraordinary public split between the presumptive White House nominee and the country's top Republican in office.
"I had friends wishing I wouldn't support him. I had friends wishing I would," Mr Ryan said.
"I really didn't feel any pressure, other than my goal is to make sure that were unified so that we're at full strength in the fall so we can win the election."
Mr Ryan's announcement, in a newspaper column published in his Wisconsin home town, marks a significant step for a party trying to come together ahead of a general election match-up against likely Democratic nominee Hillary Clinton.
And Mr Ryan made clear he had Mrs Clinton on his mind when he decided to join the ranks of Republicans who have slowly come around to backing Mr Trump, the brash billionaire few expected to emerge as the party's nominee when the campaign began in earnest last year.
"This to me is about saving the country and preventing a third progressive, liberal term, which is what a Clinton presidency would do," Mr Ryan said.
Mr Trump celebrated the endorsement on his favourite venue, Twitter.
"So great to have the endorsement and support of Paul Ryan," he wrote. "We will both be working very hard to Make America Great Again!"
However, Mr Trump made no mention of Mr Ryan during a rally in San Jose, California, on Thursday evening.
There are still some party leaders who say they will not support Mr Trump, including 2012 Republican presidential nominee Mitt Romney, who tapped Mr Ryan as his running mate four years ago.
A day earlier, Mr Romney signalled that he would support a possible third-party candidate instead of the presumptive Republican nominee.
Mr Ryan himself acknowledged that he continued to have concerns over Mr Trump's combative style, but said after a series of conversations with Mr Trump, he was confident he and the Republicans he leads as speaker would be able to work with him as president on their agenda.
"We obviously have a different kind of style and tone. That's very clear. Anyone who knows anything about us knows that," Mr Ryan said. "But what really, ultimately matters is how best can we make sure these principles and policies get enacted in 2017."
As the party's so-called "Never Trump" movement struggled to identify a viable alternative, many believed it was only a matter of time before Mr Ryan fell in line. The endorsement, he said, was not the product of any deal with the billionaire developer, but a decision based on "an understanding of our mutually agreed upon principles".
Mr Ryan said he specifically wanted to go over Mr Trump's approach to executive power, judicial appointments and his position on abortion.
"Those conversations took some time," he said, adding: "I feel much more comfortable that he's in the same page with us. Most importantly, it is obvious that Hillary Clinton is not."
Mr Ryan ended a weeks-long stand-off with Mr Trump minutes before the interview by outlining his support for the New York billionaire in a column published online by The Janesville Gazette.
He had shocked the political world last month by refusing to endorse Mr Trump once the property mogul became the last major Republican presidential contender still in the race.
The pair spoke privately in a series of Washington meetings last month and their staffs stayed in touch. Mr Ryan said he made the decision to formally endorse Mr Trump earlier in the week.
"I wasn't just going to sign up sight unseen without even having a conversation or knowing what direction, because he had only mentioned a few policies in the primary," he said.
"I wanted to basically make sure that (we agreed) on the big issues of the day, on really important principles."
Major differences remain, however. And conservative leaders across the country continue to have deep reservations about Mr Trump's devotion to Republican principles and his temperament.
In particular, Mr Ryan has embraced major changes to Medicare and Social Security as his signature issue on Capitol Hill. Most Republicans in Congress have followed Mr Ryan's plan to reduce the cost of the popular programmes that are contributing to the national debt.
But Mr Trump has repeatedly promised not to touch the popular programmes, echoing a position more commonly adopted by Democrats.
The two also disagree on immigration. Mr Trump wants to deport more than 11 million illegal immigrants in addition to imposing a temporary ban on Muslims from entering the US. Mr Ryan opposes both policies.
"It's no secret that he and I have our differences. I won't pretend otherwise," Mr Ryan wrote in The Janesville Gazette column. "And when I feel the need to, I'll continue to speak my mind."
Ryan's announcement was released as Mrs Clinton was delivering a foreign policy speech excoriating Mr Trump's approach.
Electing Mr Trump, she said, would be "a historic mistake".