Trump told Christie, who at that point was also running for president, that he "didn't expect to make it past October" and would endorse Christie when he dropped out, an unnamed Christie adviser said, according to a new book out by several members of CNN's political team.
"I think they always had an understanding that the first one out would probably endorse the other," the adviser was quoted as saying in the book, "Unprecedented: The Election that Changed Everything" that is scheduled to be released on December six.
Christie mercilessly mocked Rubio when he repeated a talking point again and again to criticise President Barack Obama.
During the Republican primary, Christie and Trump -- who have known each other through New York-area politics -- were not as harsh toward each other as were other pairs of rivals.
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Christie, though, did criticise Trump by calling on voters to get serious and not treat the election as a game, and Trump at one point alleged that Christie knew about his now-convicted aides' plans to close down the George Washington Bridge.
But on Friday, Christie was layered over as chair of Trump's transition team, with Trump's vice president, Mike Pence, now in charge of assembling the Trump administration.
The relationship between two outsized personalities will continue to unfold in the months ahead, the report said.