Rep. Dan Donovan (R-N.Y.) was flying back to Washington, D.C., from Long Island on Air Force One on a Wednesday evening last month when President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE suddenly asked about his tough primary fight against ex-Rep. Michael Grimm.

Moments later, Trump pledged his full support and said that an endorsement would come soon.

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The next week, the president tapped out two consecutive tweets warning New Yorkers that voting for Grimm, who served seven months in prison for a felony tax fraud conviction, could cost Republicans the competitive seat — and its eight-year House majority — in November.

It was a huge political gift for Donovan, who won Trump’s endorsement despite his votes against the Republican tax-cut law and legislation to repeal and replace ObamaCare.

Donovan’s cellphone immediately began “blowing up” with calls and texts from staffers, House colleagues and campaign supporters, he said, highlighting why an endorsement from Trump remains a highly coveted prize for almost any Republican.

Landing one can give a GOP candidate an enormous political boost, lots of free media and an injection of cash from Trump-aligned donors.

But Trump’s endorsement doesn’t come easy, and it often requires months of courting and the development of a personal relationship with the president.

“He is a very busy man; the North Korea summit is back on the calendar for June 12,” Donovan told The Hill in an interview. “For him to take the time after all he’s dealing with domestically and internationally to send two tweets out was incredibly special and important to my campaign.”

In Donovan’s case, the Trump tweet came after 20 years of friendship in New York in addition to a couple of Air Force One rides. He first met Trump in the mid-1990s, when he was serving as chief of staff to then-Staten Island Borough President Guy Molinari.

It was also particularly helpful given that the endorsement rebutted Grimm’s claim that he is the true Trump acolyte in the primary.

Grimm had been steadfast in making Donovan’s disloyalty to Trump an issue.

“Every time it mattered, Dan Donovan voted against President Trump,” a narrator says in one Grimm ad.

Donovan expects voters will pay more attention to Trump in the June 26 primary.

“I don’t think anyone cares what my opponent says. I think the voters care what the president said,” he said.

There is no straightforward process for winning Trump’s endorsement.

For Rep. Ron DeSantis Ron DeSantisFlorida attorney general scrutinizing Bloomberg paying fines for felons to vote Trump may meet with potential Supreme Court pick in Miami Florida governor unveils legislation targeting protesters in 'violent or disorderly' demonstrations MORE, it meant cultivating a relationship after Trump caught the Florida GOP gubernatorial candidate on Fox News, his favorite network. Trump endorsed him three days before Christmas, before DeSantis had even jumped into the race.

Last week in Tennessee, Trump raised cash for GOP Rep. Marsha Blackburn Marsha BlackburnTaylor Swift on National Voter Registration Day: 'We need everyone' Democrats smell blood with new DHS whistleblower complaint Hillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections MORE, then appeared with her at a Nashville rally while ripping her likely Senate opponent, former Democratic Gov. Phil Bredesen, as an “absolute tool” of Senate Minority Leader Charles Schumer Chuck SchumerCruz blocks amended resolution honoring Ginsburg over language about her dying wish Senate Democrats introduce legislation to probe politicization of pandemic response Schumer interrupted during live briefing by heckler: 'Stop lying to the people' MORE (D-N.Y.).

Trump also tweeted his endorsement of California Republican businessman John Cox in a bid to help him consolidate GOP support and win one of two spots on the November ballot for governor. In Tuesday’s “jungle” primary, the top two candidates will advance, no matter their party.

In Ohio, the president invited GOP Rep. Jim Renacci James (Jim) B. RenacciOhio is suddenly a 2020 battleground Democrats fear Ohio slipping further away in 2020 Medicare for All won't deliver what Democrats promise MORE to sit next to him at a Cleveland tax roundtable and praised the congressman. Renacci won his Senate primary days later and will face incumbent Sen. Sherrod Brown Sherrod Campbell BrownHarris faces pivotal moment with Supreme Court battle Remote work poses state tax challenges Senate Democrats release report alleging Trump admin undermined fair housing policies MORE (D) this fall.

While he has a political team at the White House, it’s Trump who largely calls the shots about which candidate to endorse or campaign with, say those familiar with the process.

Trump likes Republicans who stood by him early in the 2016 GOP presidential primary. Renacci, Blackburn and Rep. Lou Barletta Louis (Lou) James BarlettaBottom Line Ex-GOP congressman to lead group to protect Italian products from tariffs Head of Pennsylvania GOP resigns over alleged explicit texts MORE (R-Pa.), who is running for Senate, were among the first House lawmakers to endorse Trump and served as surrogates for him on the campaign trail.

“It’s pretty clear cut why Trump endorses people like Renacci and Barletta, because they were with him early,” said a former senior GOP aide.

In Florida, DeSantis relied on his years-long relationship with Trump to win support. Trump had endorsed the former Navy lawyer and Tea Party favorite in his first House race in 2012.

DeSantis has been a fixture on Fox News, where he has earned a reputation as a staunch Trump defender and fervent critic of special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s investigation.

He asked Trump for his support in the governor’s race last December as he sat across from the president on an Air Force One trip to a campaign rally in Pensacola, Fla., according to GOP sources familiar with the conversation.

The president briefly inquired about DeSantis’s top primary challenger, state Agriculture Commissioner Adam Putnam, then said he was “all in,” the sources said.

It would be great if Trump could tweet the endorsement, DeSantis followed up. Trump agreed.

A few weeks later, at the Dec. 22 White House bill signing of the tax-cut legislation, Trump spotted DeSantis and told him “heads up,” the tweet would be coming shortly.

“Congressman Ron DeSantis is a brilliant young leader, Yale and then Harvard Law, who would make a GREAT Governor of Florida. He loves our Country and is a true FIGHTER!” read the tweet, sent a few hours later.

Donovan, 61, said that Trump was not blindsided by his negative votes on ObamaCare and the tax-cut bill. Donovan opposed the tax bill’s elimination of the state and local tax deduction. Even when it was partially restored with a $10,000 ceiling, Donovan said it would leave too many of his constituents paying more in taxes.

Asked if Trump was upset by his “no” votes, Donovan replied: “He didn’t hold a grudge at all,” even though the congressman’s past “positions” were discussed during the Air Force One ride late last month.

“The cap of $10,000 does not help the people I represent, and the president understands this,” Donovan said. “He has to look at what’s good for 330 million people; I have to look at what’s good for the 740,000 who I represent.”

Trump’s tweet backing Donovan did cause some confusion, since the president said Donovan had voted for “tax cuts.”

Donovan explained that Trump was referring to his past support for extending the Bush tax cuts and making permanent a number of tax breaks under the 2015 PATH Act. On Monday, PolitiFact said Trump’s assertion that Donovan voted for tax cuts was “mostly true.”

Perhaps to clear things up, the Trump campaign issued a press release on Monday touting the Donovan endorsement, but mentioning neither health care nor tax reform.

“It’s very clear to the president and many voters in our district,” Donovan told The Hill, “that if our opponent were to win the primary, [Republicans] would lose the general election and the only Republican seat in New York City.”