Why?

GOP strategist Castellanos: Time to rally around Trump, ‘too late to ask mommy to step in and rewrite rules’
By Tom Hamburger and Matea Gold
March 2
"If Trump is going to be our nominee, as I believe he is, it is our mission to support Trump and make him the best nominee and president possible."

Lindsey Graham: "We're gonna lose to Hillary Clinton" with Donald Trump
By Rebecca Kaplan
CBS News
March 2, 2016
"Ted Cruz is not my favorite by any means and I don't wish him ill,...but we may be in a position where we have to rally around Ted Cruz as the only way to stop Donald Trump and I'm not so sure that would work," he [Graham] said.

It's not clear from "establishment" GOP critics whether they fear Trump because...

They think he'll lose to Hillary...
... or he'll shred the GOP...
... or he'll make a horrible President.

... or perhaps some combination of these three.

With the exception of the now-clearly-delusional Rubio supporters, the "establishment" folks seem to have come to the realization that their Hobson's choice is between Trump and the equally loathsome Cruz.

Why can't they be honest (AND patriotic) and simply say, "We messed up. A loyal Republican American ought to stay home come November."

Heck - it may even be strategically wise:

1. Avoids the possibility of a DISASTROUS GOP Presidency.
(... from which Dems would benefit in 2020.)

2. Offers the possibility of a DISASTROUS Democratic Presidency from which GOP can benefit come 2020!

No one likes Trump (well, no one except his extremely energized supporters - currently less than 50% of the GOP!).
No one likes Cruz.
Why is it necessary to support either of 'em?

Just askin'.