Nehlen has forced Ryan to flip-flop on the Trans Pacific Partnership (TPP) trade deal, which he previously supported but now opposes, and has forced Ryan for the first time in over a decade to campaign and respond to criticisms in his district.
"He's not a war hero", Mr Trump said in July 2015 at a gathering in Ames, Iowa, of religious conservatives.
Ryan reiterated his support for Trump hours before the endorsement, but the Wisconsin Republican noted his support wasn't a "blank check" and pledged to speak out against the businessman's divisive positions if necessary.
Donald Trump has ended his political stand-off with Paul Ryan, endorsing the US House speaker in a speech on Friday.
"He's a good man and he's a good guy".
Party leaders no doubt hope that Trump's endorsements Friday will mark a pivot on the part of the nominee to get serious and back off from the barrage of alienating comments he has made since entering the presidential race.
Trump, whose candidacy has been marked by frequent attacks on Republican leadership, changed his tune from just days earlier.
"And while I'm at it, I hold in the highest esteem Sen. John McCain for his service to our country in uniform and in public office, and I fully support and endorse his re-election", the real estate mogul said Friday.
"We need unity. We have to win this election", Trump told a rally in Ryan's state of Wisconsin, as he stressed a "big tent" Republican Party is the only way to defeat Democrat Hillary Clinton in November's election.
Trump heavily emphasized his outsider status in the lead-up to delivering his endorsements of three incumbent Republicans with a combined tenure in Congress of more than a half century.
Trump's moves came after many Republicans urged the candidate to correct course following a tumultuous week. Wisconsin Assembly Speaker Robin Vos offered a blunt message ahead of the presidential nominee's arrival: "We are Ryan Republicans here in Wisconsin, not Trump Republicans". "When we do that, we're comfortable that we can get the agenda and the narrative of the campaign back on where it belongs, which is comparing the tepid economy under Obama and Clinton, versus the kind of growth economy that Mr. Trump wants to build". Trump said he told Pence he could endorse Ryan if he wanted, even while Trump himself demurred.
It's been a long journey for Trump, however, who told the Washington Post on Tuesday: "I like Paul, but these are disgusting times for our country". And I'm just not quite there yet.
Trump's approach to national security came under fire Friday as well, with former CIA Director Michael Morell contending the Republican nominee would make "a poor, even risky commander in chief".
Former New York City Mayor Rudy Giuliani told NJ Advance Media that Trump is set to alter his campaign to focus more on the "positive than the negative". But he also stressed the importance of unifying the GOP.
The former secretary of state sought to "clarify and explain" a recent statement on "Fox News Sunday" that FBI Director James Comey said her answers to the bureau about her use of a private email server were "truthful".