Of course, any highly public relationship like the one between Mr. Trump and Mr. Christie can make for awkward situations. As United States attorney, Mr. Christie went after Charles Kushner, a real-estate developer and major New Jersey political donor who pleaded guilty in 2005 to tax evasion and making illegal campaign contributions — and four years later became the father-in-law of Mr. Trump’s daughter Ivanka.

But by 2010, Mr. Trump was then battling with another rich New Yorker, Carl Icahn, to win back control of his Atlantic City casinos after they entered bankruptcy. He won a month after Mr. Christie’s inauguration. A year later, the governor, Mr. Trump and their wives landed on Page Six, in The New York Post, after another dinner at Jean-Georges. Mr. Icahn became a backer of Mr. Trump for president; Mr. Trump suggested he might appoint him Treasury secretary.

Mr. Trump made a larger-than-usual donation, of $250,000, to the Republican Governors Association in 2014, when Mr. Christie led it. Mr. Kushner, too, has held at least one event for Mr. Trump at his home on the Jersey Shore.

“It started out professional, but I think it’s definitely evolved into a more personal relationship,” Dale Florio, a longtime Republican fund-raiser in New Jersey, said of the Trump-Christie connection.

Others who know the two men describe their alliance more as an acquaintanceship, as many of Mr. Trump’s relationships are. His true friendships are limited to the small number of people he plays golf with in Palm Beach or at Trump National Golf Club Bedminster, which he created from the former estate of John DeLorean, the flamboyant auto industry executive, in the heart of New Jersey horse country. (Mr. Christie is not a golfer.)

Once they started to compete to be the Republican nominee, the friendship became strained.

As Mr. Trump surged in the polls, one New Jersey political leader, who like many people interviewed for this article did not want to be identified out of fear of reprisals from either man, said he had urged Mr. Christie to directly confront Mr. Trump, saying he was the only candidate who could do it. Mr. Christie did not disagree, this person said, but “he was afraid to do it — he’s never been afraid of anybody.”