__________UPDATE__________
While Mitt Romney has yet to endorse any candidate, and mutuality of favors is not much in politics, there is this paragraph from within a Feb. 2, 2012, NY Times report of a splash of a show in Nevada where Trump endorsed Mitt, apparently at a Trump-owned hotel:
When it was Mr. Romney’s turn to speak, Mr. Trump assumed his place next to Mrs. Romney, and they stood together as an adoring couple joined in the service of Mr. Romney. While the Trump default face is one of pure satisfaction, the maven looked every bit the proud father when the candidate lauded “Donald Trump’s magnificent hotel.” As always, Mr. Romney reminded everyone that he had spent his life in the private sector — but added the caveat that he was “not quite as successful as this guy,” and Mr. Trump nodded the slow nod of a great man for whom respect had been paid.
Apart from whether Trump's endorsement then really mattered, and whether a Romney endorsement of anyone this cycle would really matter; good theater is good theater, when the acting is good.
FURTHER: An interesting link. This mid-item paragraph, with embedded links omitted:
Well, that was then. Rubio is no more likely to escape the video of his catastrophic Saturday night massacre than Edmund Muskie did his (alleged) tearing up during the 1972 New Hampshire primary, Michael Dukakis did his photo op in a tank, Howard Dean did his scream, or Perry did “Oops!” The only stock that is rising for Rubio is his status as a national laughingstock. It was particularly ill-advised of him to attack Joe Biden at one point in the debate: America knows Joe Biden, and Rubio is no Joe Biden. He’s the new Dan Quayle.
__________FURTHER UPDATE__________
Casino mogul Sheldon Adelson has yet to endorse a candidate, playing his cards close to his vest for now, but his Vegas newspaper did send up a puff of white smoke, for what it was worth; this link.
The Feb. 5 Review Journal oped touting Rubio made no analogies to Dan Quayle; nor to Joe Biden either for that matter. And Trump stood them up:
When we began this process, we made it clear to candidates that they had to meet with us to gain consideration for our endorsement. We made many attempts to meet with businessman and Republican front-runner Donald Trump, but he could not work an interview into his schedule.
Anyone who declines a shot at Sheldon Adelson's favor puts backing behind a claim that he does not solicit money from others and wants no obligations pinned to him from accepting money. That remains in Trump's favor; but candidates such as Rubio are incapable of self-financing a presidential shot, and would be out of the game without taking such money.
At this point in time, the Clintons could self finance, but so far appear to decline doing so.
___________FURTHER UPDATE___________
While having stood up Adelson's newsmen, Trump has shared a bit of his sunshine, time being available for Rudy.