Trump has threatened the judge on his case that he might sue him after he's President.
http://www.nytimes.com/2016/06/04/us/politics/donald-trump-constitution-power.html?ref=politics
Donald Trump Could Threaten U.S. Rule of Law, Scholars Say
And, in what was a tipping point for some, he attacked Judge Gonzalo P. Curiel of the Federal District Court in San Diego, who is overseeing two class actions against Trump University.
Mr. Trump accused the judge of bias, falsely said he was Mexican and seemed to issue a threat.
They ought to look into Judge Curiel, because what Judge Curiel is doing is a total disgrace, Mr. Trump said. O.K.? But we will come back in November. Wouldnt that be wild if I am president and come back and do a civil case?
David Post, a retired law professor who now writes for the Volokh Conspiracy, a conservative-leaning law blog, said those comments had crossed a line.
This is how authoritarianism starts, with a president who does not respect the judiciary, Mr. Post said. You can criticize the judicial system, you can criticize individual cases, you can criticize individual judges. But the president has to be clear that the law is the law and that he enforces the law. That is his constitutional obligation.