I think Trump's position is that this particular judge, with Mexican heritage, is so much against Trump's build-the-wall and no illegal immigration policy positions, that this particular judge with Mexican heritage is not impartial toward Trump.
But that is not what he says.
The press isn't very skilled at teasing this out. It could ask "Is there any judge with Mexican heritage that you would accept?" But, it has confirmation bias, in that it assumes Trump is racist, so whatever objection he has, has a racist motivation.
Trump does not start with the premise that the Mexican judge cannot be a bigot because he is Mexican. That is exactly what this judge is if he belongs to or defends La Raza (The Race).
It’s not that difficult for Trump to point out the Judge’s connection to La Raza and an example or two of actions by the court that are biased. Also, in agreement with Newt, I would parade a half dozen Trump University grads that endorse the courses and have been successful.
That may or may not be his position but he has no evidence that this particular judge is either for or against the wall, which wasn’t even an issue when the case started. Which is most likely why his lawyers haven’t gone through the proper procedure and filed a petition for reassignment.
Trump needs to connect the dots:
La Raza Judge...(dots)...Violent La Raza "protesters".......Killary.....La Raza Judge