Their friendship began after an introduction through Donald J. Trump’s sister. It ended nearly 20 years later, when Mr. Trump refused to concede the 2020 election to Joseph R. Biden Jr.

In between, Chris Christie, the former governor of New Jersey, and Mr. Trump had a relationship that could be genuinely warm, with chats about politics and current events, and at other times transactional.

Mr. Christie gave Mr. Trump a boost by endorsing his 2016 candidacy after ending his own bid for the Republican nomination, and then coached him for debates and led his initial presidential transition team. In return, Mr. Trump passed him over for the roles of vice president and attorney general.

Mr. Trump eventually turned back to Mr. Christie for other advice during his term. But by the midway point of the presidency, Mr. Christie seemed content to be on the outside.

Their last exchange was in August 2021, according to a person briefed on the matter, when the former president had an aide send Mr. Christie a testy message.

Now, they have entered a new chapter: open hostility. Mr. Christie announced his second presidential campaign on Tuesday in New Hampshire, aiming to stop Mr. Trump from a second term in the White House.

“I think he’s a coward and I think he’s a puppet of Putin,” Mr. Christie, speaking recently to the conservative radio host Hugh Hewitt, said of the man he once supported.

Here’s a look back at how their relationship grew, thrived and then wilted.

Casual Acquaintances, Then Presidential Rivals

Mr. Christie was a United States attorney in New Jersey, where Mr. Trump still had casinos, when the two men first dined together.

That May 2002 introduction over dinner came through an intermediary, Maryanne Trump Barry, Mr. Trump’s older sister, who was a federal judge in the state at the time and described Mr. Trump to Mr. Christie as “my little brother.” In Mr. Christie’s 2019 memoir, “Let Me Finish,” he wrote about his first impressions of Mr. Trump, who in two years would begin his run as the star of the reality TV show “The Apprentice.”

“Donald was opinionated,” Mr. Christie wrote. “He was bombastic. He was entertaining. He talked about his business with infectious enthusiasm and considerable detail. I came away with the impression that public Donald and the private Donald were pretty much one and the same.”

It was soon clear that Mr. Christie could end up as a candidate for governor someday. He won the office in his first attempt, in 2009, two years before Mr. Trump considered running for the White House against President Barack Obama.

Both men knew each other in the way that prominent people in the New York media market tend to: casually, with paths that periodically crossed.

In 2015, both Mr. Christie and Mr. Trump ended up declaring presidential candidacies.

Mr. Christie, by then hobbled by the “Bridgegate” political retribution scandal, had nonetheless fashioned a national political brand as a straight-talking candidate.

By contrast, some viewed Mr. Trump as a sideshow who would eventually fade, even as he was leading in the polls. At the time, Mr. Trump told Mr. Christie privately that he didn’t expect his campaign to last beyond October 2015.

Their relationship began to be tested. Two months after Mr. Trump’s entrance into the race, Mr. Christie told Fox News that the New York businessman didn’t have the “temperament” or experience to be president. Mr. Trump taunted Mr. Christie for being absent from New Jersey, where he was still governor.

Ultimately, Mr. Trump overshadowed his newfound rival — and all other rivals — with an endless stream of inflammatory pronouncements, including a proposal to ban Muslims from entering the country.

Mr. Trump saved his most hostile barbs for candidates other than Mr. Christie. In turn, the governor trained his most aggressive fire on Senator Marco Rubio of Florida during a debate in New Hampshire shortly before the state’s primary, mocking him for a “memorized 25-second speech.”

But after staking his candidacy on New Hampshire, Mr. Christie finished a dismal sixth and dropped out of the race.

A Key Ally, Up to a Point

When Mr. Trump won the South Carolina primary, Mr. Christie told allies the writing was on the wall — it was clear Mr. Trump was on track to become the nominee.

“I am proud to be here to endorse Donald Trump for president of the United States,” Mr. Christie said at an endorsement event in Florida in February 2016, as astonished reporters watched him praise Mr. Trump’s candidacy. After Senator Jeff Sessions of Alabama, Mr. Christie was one of the first prominent Republicans to endorse Mr. Trump at a time when the party’s leadership was still trying to stop his ascent.

Soon, Mr. Christie was a key adviser to Mr. Trump. He was also for a time considered as a potential running mate, but some of Mr. Trump’s advisers, including members of his family, argued against it. (Mr. Christie had also prosecuted the father of Mr. Trump’s son-in-law Jared Kushner years earlier, and Mr. Kushner was opposed to the selection of Mr. Christie.)

Mr. Trump ultimately chose Mike Pence, then the governor of Indiana, who had been introduced to Mr. Trump through Mr. Christie.

Mr. Trump tried to keep Mr. Christie on the hook, the former governor wrote in his memoir, insisting in a phone call to Mr. Christie that he hadn’t decided on his running mate yet, even as he made plans to fly Mr. Pence to New York for a news conference.

Mr. Christie led Mr. Trump’s preparations for the general-election debates against Hillary Clinton, the Democratic nominee. But after the October 2016 release of a recording in which Mr. Trump described grabbing women by their genitals, Mr. Trump privately griped that Mr. Christie had not more vocally backed him.

Mr. Christie also served as the head of his transition team, a job from which he was dismissed shortly after Election Day by Mr. Kushner; Stephen K. Bannon, the chief strategist for Mr. Trump; and Reince Priebus, who would become Mr. Trump’s first White House chief of staff.

Behind the Scenes, Distancing

Mr. Trump asked Mr. Christie to lead a task force on opioids, an issue Mr. Christie had been concerned about as governor. Mr. Christie was also said to be a personal favorite of Mr. Trump’s wife, Melania.

But Mr. Trump decided against giving him the job of attorney general, which went to Mr. Sessions. Instead, Mr. Christie has said, the president offered him various roles at different points, including labor secretary and secretary of the Homeland Security Department.

Mr. Trump also took a suggestion from Mr. Christie as to who could replace the fired F.B.I. director, James A. Comey. It was Mr. Christie’s lawyer during the Bridgegate scandal, Christopher A. Wray, who was appointed and remains atop the agency. Mr. Trump soon started complaining that Mr. Wray was not doing what he wanted at the agency, and blamed Mr. Christie for a nomination that Mr. Trump had put forward.

Mr. Christie took himself out of consideration to succeed John F. Kelly as Mr. Trump’s chief of staff at the end of 2018 after Mr. Trump had offered the job to Mr. Christie. By then, it had become clear that Mr. Trump was cycling through staff members and firing them at a rapid clip.

In February 2020, Mr. Trump pardoned a former software chief executive whose clemency Mr. Christie had lobbied for.

That year, Mr. Christie wrote Mr. Trump a lengthy memo instructing him how to handle the coronavirus pandemic. It was ignored.

Mr. Trump brought Mr. Christie in for debate preparations once again, and some of his aides faulted Mr. Christie when Mr. Trump’s initial debate against Mr. Biden was disastrous. (Mr. Trump appeared physically unwell at the debate and may have already been affected by the coronavirus; the news of his Covid diagnosis came days later.)

When both Mr. Trump and Mr. Christie were hospitalized with serious bouts of Covid shortly after that debate, Mr. Trump called his debate coach at the hospital. “Are you going to say you got it from me?” Mr. Trump asked Mr. Christie, the former governor later recounted in his second book, “Republican Rescue.” They both recovered, but Mr. Christie made clear he thought he should have worn a mask at the prep sessions, angering Mr. Trump.

Breakup

Hours after Election Day ended, when Mr. Trump delivered a speech claiming widespread fraud, Mr. Christie, by then a contributor for ABC News, said on air that Mr. Trump needed to offer proof.

In an interview with The New York Times in November 2022, Mr. Christie said he had last spoken with Mr. Trump in December 2020, after the president saw him deride Mr. Trump’s legal team on television. Mr. Christie told him he should concede the election to Mr. Biden and host the president-elect in the White House.

“He told me he would never, ever, ever, ever do that,” Mr. Christie said. “And that was the last time we spoke.”

In 2021, Mr. Trump described Mr. Christie as an “opportunist” to a reporter. Four months later, he had an aide send Mr. Christie a printout of a tweet by Mr. Christie related to the pardon that he had sought for the former software executive. “Chris,” he wrote, according to the person briefed on it, “How quickly people (some) forget – Best Wishes,” with his signature.

Mr. Christie responded cordially, wishing Mr. Trump well.

Shane Goldmacher contributed reporting.