How do they threaten or coerce him?
by saying they wont finance his next election?
“How do they threaten or coerce him?”
By saying they are going to cut him off and shift their money to Hillary.
That was to answer your question.
Aside that,
One of the things Trump and his supporters always said was he funded himself and was therefore his own man. He was not sold out or beholden to anybody.
That was his big selling point.
He can’t say that now.