"It's a really serious charge, against a judge," said Professor Susan Low Bloch of Georgetown Law. "The court does not take it lightly. And therefore lawyers are well-advised to be very careful with hurling those accusations."
The presumptive Republican nominee last week called U.S. District Court Judge Gonzalo Curiel "a hater of Donald Trump," based on his rulings so far, and pointed out that one of Trump's policies is to build a wall to stop illegal immigration, and the judge is of Mexican heritage. "I'm building a wall. It's an inherent conflict of interest," Trump told the Wall Street Journal.
But complaints about a judge based primarily on ethnicity have found little traction in the past, and have even gotten lawyers into hot water.
For example, in a 1986 case
, Baltimore defense attorney Paul Evans was banned from the court and recommended for disbarment after he complained in a letter that the judge in his case "either was grossly incompetent or biased in favor" of what he called "the Jewish firm" involved in the proceedings.
An appeals court upheld the punishment, saying the letter was "undignified, discourteous, and degrading" and that Evans never demonstrated the judge's incompetence or bias using examples or evidence.
And in 1996, attorneys Larry Klayman and Paul Orfanedes questioned the fairness of U.S. District Court Judge Denny Chin. They pointed out in a letter that the judge was an appointee of President Bill Clinton and an Asian-American, who they claimed could be biased due to Klayman's involvement in litigation against Democratic donor John Huang and other Asian-Americans.
Chin said to Klayman, "You asked questions of the Court, at least in part, because of my race?"
"In part," Klayman responded. "I, for instance, would not sit as a Jewish American on a case that involved a Palestinian."
Chin said the question was "offensive." He ordered Klayman and Orfanedes never to appear before him again, and to notify any other judges they appeared before that they had been sanctioned.
An appeals court upheld the punishment in 1998, saying, "A suggestion that a judge cannot administer the law fairly because of the judge's racial and ethnic heritage is extremely serious, and should not be made without a factual foundation going well beyond the judge's membership in a particular racial or ethnic group."
If Trump's lawyers do challenge Curiel on his ethnicity, one lawyer who has known the jurist for 20 years says he might take it in stride.
"He's not going to fly off the handle," said criminal defense attorney Jeremy Warren. "He's a calm, deliberate, contemplative judge."
But experts warn the move could be risky.
"It's a huge insult," said Bloch. "If it's unfounded, then we have examples where lawyers have been sanctioned."
Trump's attorneys in the case did not respond to inquiries from CNN. But one of them, Daniel Petrocelli, said a month ago, "We're not seeking to recuse the judge." He told Yahoo News
, "The judge is doing his job."
In a statement Tuesday night
, Trump shifted focus, saying his concerns were not just the judge's ethnicity, but also the fact that he was appointed by President Barack Obama, and has "reported associations with certain professional organizations." Curiel is a lifetime member of the National Hispanic Bar Association, which last year called for a boycott of all Trump business ventures -- although it is not clear whether Curiel personally agrees with the boycott.
If Trump's lawyers were to seek the judge's recusal, according to defense attorney Danny Cevallos, under federal law they would have to either show the judge "has a personal bias or prejudice... against him or in favor of any adverse party," (28 U.S.C. § Section 144), or show the judge has "a personal animus or malice... of a kind that a fair-minded person could not entirely set aside." (Section 455.)
The next step would be to provide evidence of bias, Cevallos said
: "If, for example, Judge Gonzalo Curiel started tweeting #NeverTrump last night, or showed up in late May to heckle at a Trump event."
But Curiel's membership in a Latino bar association, and his appointment by a Democratic president, according to Bloch, would likely not be grounds for recusal. "If you show that the judge historically has said disparaging things, or if you could show that he consistently rules against certain kinds of people, you might be able to make some kind of case," she said. "But you need real facts."
In his statement Tuesday night, Trump said he does not believe that a person's heritage alone makes them incapable of being impartial, and his comments have been misconstrued as an attack against people of Mexican heritage.
"But, based on the rulings that I have received in the Trump University civil case," he said, and "given my unique circumstances as nominee of the Republican Party and the core issues of my campaign that focus on illegal immigration, jobs, and unfair trade, I have concerns as to my ability to receive a fair trial."
Former Attorney General Alberto Gonzales told CNN's Ashleigh Banfield that Trump has a right to ask if Curiel is unbiased, because every American citizen in entitled to a fair trial before an impartial judge.
"If there any questions related to that impartiality -- even the appearance of impropriety is enough to cause a judge to recuse himself -- then there should be recusal," he said.
But Gonzales was quick to add that he does not believe the judge in this case should be recused, and said, "the fact that Donald Trump has attacked a Hispanic judge, to me, makes me angry."