He went on:
"If our self-indulgent Republican party establishment had really wanted to prevent a takeover of the GOP, they should not have gorged on political power while they failed to do anything to prevent the decline of the country. Our leaders could have led. They could have done more than say 'no' to Democrats while offering no alternative.
"They should have stood up for the change Donald Trump is bringing now but they didn’t.
Now, Trump has earned the nomination. He won it, fair and square and we should respect that. Donald Trump whipped the establishment and it is too late for the limp GOP establishment to ask their mommy to step in and rewrite the rules because they were humiliated for their impotence.
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."
Castellanos, who said he has no interest working for the real estate mogul's campaign, said the reality of Trump's momentum is setting in across the party.
"I think it will soon be clear that Republicans have hired Donald Trump to do the job their previous hires have failed to do: bring change to Washington and make America great again," he wrote.