Mr Trump would praise Mr Christie for his takedown of Mr Rubio.
New Jersey Governor Chris Christie's February endorsement of US President-elect Donald Trump shocked observers as the first sign of capitulation from the Republican establishment to the man who would eventually become their nominee -- and then the US President.
It could've easily been the other way around. That's one revelation in "Unprecedented: The Election that Changed Everything," CNN's upcoming book on the 2016 race that comes out December 6.
"Trump told Christie last year that he didn't expect to make it past October -- at which point he would endorse Mr Christie, according to a Christie adviser who asked not to be named in order to speak about behind-the-scenes maneuvers," according to the book.
"I think they always had an understanding that the first one out would probably endorse the other," the adviser said.
During the throes of the Republican primary, Mr Trump and Mr Christie had a common enemy: Marco Rubio.
After Mr Christie's endorsement, Mr Trump would praise Mr Christie for his takedown of Mr Rubio during the Republican debate right before the New Hampshire primary.
Christie mercilessly mocked Mr Rubio when he repeated a talking point again and again to criticise President Barack Obama.
During the Republican primary, Mr Christie and Mr Trump -- who have known each other through New York-area politics -- were not as harsh toward each other as were other pairs of rivals.
Mr Christie, though, did criticise Mr 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.
Mr Christie has since become one of Mr Trump's most stalwart defenders, and he is currently under consideration for a number of high-profile cabinet posts.
But on Friday, Mr Christie was layered over as chair of Mr Trump's transition team, with Vice President-elect Mike Pence, now in charge of assembling the Trump administration.
The relationship between two outsized personalities will continue to unfold in the months ahead.