Why Rubio hasn't endorsed Cruz

“At this point, No. 1, it’s not a year where endorsements matter very much,” Rubio told POLITICO. “And No. 2: I’ll just let it play itself out and work through it. I really want to come back and focus 100 percent on my work in the senate.”

Rubio has already helped Cruz by renting him his fundraising list, calling him “the only conservative left in the race,” withdrawing his name from the primary ballot in select states so as not to siphon votes from Cruz and petitioning to hang on to some of the 171 delegates he won to keep them from going to Donald Trump…

“Marco wants Donald to lose. If he thought his endorsement would help in California or in Indiana, which it won’t, then he would probably do it,” the source said. “But what Marco isn’t going to do is just endorse Ted, watch Trump win anyway and then, in four years, watch Cruz use Marco’s endorsement against him if they both run for president again,” the source said…

“A lot of our donors haven’t done much for Cruz because they don’t really see him winning anyway. Many of them don’t really like him. And they think like, with Marco’s endorsement, it really won’t do much. And some of them have told him that Cruz would just use the endorsement as a cudgel against him,” another Rubio insider said. “There’s no gain.”