There is a growing sense among political pundits, especially of the establishment class, that only Ted Cruz has a reasonable shot at stopping Donald Trump. Jeb Bush has already achieved terminal velocity, Christie and Kasich have been unable to break the top 3 in New Hampshire, and Fiorina has been demoted from the debate main stage. Rubio, who was their 6th or 7th choice, has yet to achieve the same kind of liftoff that Cruz has, and he doesn't have an obvious strategic path to victory like Cruz does.
When faced with a choice between Trump and Cruz, the establishment chooses Trump. The first evidence of this was when Reince Priebus steadfastly refused to admit what even Debbie Wasserman Schultz admits, that Ted Cruz is eligible to run for President. The way the rest of the establishment, including John McCain, have failed to defend Cruz on the completely meritless birther issue shows that they are wholly willing to throw Cruz to the wolves and take their chances with Trump.
Further, the two most genuinely establishment pundits on the right - Laura Ingraham and Ann Coulter - have been in the tank for Trump since basically day one. As much as Ingraham regularly blasts the establishment on her twitter, and the insulated thinking of the "New York-Washington corridor," she has basically the most establishment pedigree humanly possible. Ingraham was born and raised in Connecticut. She went to Ivy league undergrad and Virginia law. The only non-political job she has ever had was working as a big law associate in Manhattan. She has worked in and around the beltway for her entire adult life, much of which has been spent earning a very comfortable salary as a classically establishment journalist. Nothing says "establishment" like being a regular fill in for Bill O'Reilly's makeup.
Coulter, likewise, has been a tool of the establishment since she burst onto the scene in the 90s as part of the anti-Clinton attack machine (a role which she admittedly played very well). She, however, likewise spends virtually all of her time shuttling between New York and DC - in fact, one of the most interesting chapters of any of her books was her comparison of the dating scene in DC as opposed to New York. She was an extremely early Romney adopter in both 2008 and 2012, and her establishment bona fides were pretty well cemented with her condescending lecture to anyone who thought replacing Thad Cochran with Chris McDaniel might be a good idea.
If you need more convincing, here is Alex Castellanos making the case explicitly.
Why would the establishment prefer Trump, the man they have fought against so vociferously since the day he announced? Well, they know that, bluster aside, not much would probably change under a Trump presidency. Trump has no special interest in cutting spending or eliminating special interests. Given that he's been the beneficiary of both throughout the entirety of his career in private industry, they have reason to believe that under a Trump presidency, the trough will only get wider and deeper while Trump is off chasing after a border wall they know they can prevent him from building. And ultimately, if Trump loses (as they expect him to do), then they can just fundraise off anti-Hillary angst for at least the next four years. Not the worst possible result, from the standpoint of a guy like Mitch McConnell.
On the other hand, they have reason to believe that if Cruz is elected President, he might actually try to do some of the things he is campaigning on. The history of their interaction with him shows that he means business about trying to cut spending, and actually overturning Obamacare - a proposition which scares them half to death.
In the end, they are not really bothered by the prospect of a guy who allegedly converted to conservatism five minutes ago and shows no real ideological mooring other than relentless self-promotion. They already swallowed that pill when they backed Romney in 2012 and McCain in 2008, after all.
So faced with a choice between the two, the establishment has clearly taken sides and their side is Trump's.