In the week before the Florida Republican primary in March, Lee headed early to the GOP presidential debate there with a mission: to convince one Senate friend, Rubio, to ally with another, Ted Cruz, and form a unity ticket to block Donald Trump from winning the Republican nomination.
But at the last minute, as Lee prepared to board his plane, Rubio backed out of the meeting.
That's another revelation in "Unprecedented: The Election that Changed Everything," CNN's upcoming book on the 2016 race that comes out December 6. It was written by CNN's Thomas Lake with reporting from Jodi Enda, Susan Baer and CNN's political team.
"Cruz was serious enough about the alliance that he authorized Sen. Mike Lee of Utah to go to Miami ahead of the CNN debate there on March 10 to help work out a deal," the authors write.
"I think the two of them as running mates would have been unstoppable," Lee later told CNN. "They would have united the party."
The battle between Cruz and Trump became increasingly bitter as the election progressed, with Trump repeatedly attacking Cruz's family and Cruz eventually withholding his endorsement from Trump at the Republican National Convention. Cruz did eventually give him his nod of approval in September, and he is now a possible attorney general nominee.
Though Rubio was initially open to the unity idea, according to two sources familiar with the talks, he declined the meeting with Lee, who bought the ticket at his own expense.
"I don't think I can do this," Rubio told Lee, according to one of the sources. "I don't think I can back out and not be a presidential candidate prior to the primary in Florida."
And so he didn't. Five days after the debate, Trump won Florida by 18 points and Rubio immediately dropped out of the race.
He never endorsed Cruz. And Cruz never was able to keep Trump from becoming the Republican nominee.