Donald Trump wants to disqualify a judge purely because of his ethnicity

After months of underhanded attacks on Judge Gonzalo Curiel, the federal judge presiding over two class-action suits against Trump University, Donald Trump finally came out and said it: people with "Mexican heritage" can't be trusted to put American values first.
Here's what he told the Wall Street Journal on Thursday:

Mr. Trump said U.S. District Judge Gonzalo Curiel had "an absolute conflict" in presiding over the litigation given that he was "of Mexican heritage" and a member of a Latino lawyers’ association. Mr. Trump said the background of the judge, who was born in Indiana to Mexican immigrants, was relevant because of his campaign stance against illegal immigration and his pledge to seal the southern U.S. border. "I’m building a wall. It’s an inherent conflict of interest," Mr. Trump said.

Repeatedly and over months, Trump and his campaign have insinuated that Curiel is hostile toward Trump and everything he represents because he's Hispanic and proud of it. And now, Trump is doing us all the favor of stating it outright.

Related Donald Trump’s smear campaign against a federal judge, explained

Donald Trump doesn't say outright that there are "good" and "bad" Hispanics in America. He doesn't have to. When in one breath he says "I love the Hispanics" and "the Hispanics will vote for me" and in the next he says that someone with "Mexican heritage" can't be objective in judging a case against him because they oppose his building a wall, his supporters can fill in the blanks.
This isn't the first time the Trump campaign has implied that people who aren't white men are incapable of being objective: Campaign chair Paul Manafort all but ruled out the idea of nominating a nonwhite or nonmale vice president because it would "be seen as pandering."

But what Trump is doing now is more sinister: He's implying that taking pride in one's ethnicity — to the extent of, say, being a member of an ethnic association — indicates that you're not proud of being an American. He's nonchalantly calling, by extension, the patriotism of millions of nonwhite Americans into question.

Judge Curiel is seen as the "wrong kind" of Hispanic — the disloyal kind

By any objective measure, Gonzalo Curiel ought to be the kind of Hispanic whom Donald Trump likes a lot. He spent years as an assistant US attorney in San Diego, prosecuting people for smuggling drugs over the border. He's literally gotten death threats from a member of a Mexican cartel. Judge Curiel has done more to secure the border over his career than Trump ever has.
But Curiel refused to dismiss the class-action lawsuits against Trump University, even though Donald Trump thought he should. That makes him suspect. And because he's insufficiently supportive of Trump, it stands to reason that he must be the wrong kind of Hispanic — the kind who doesn't care about the border.

Judge Curiel has done more to secure the border over his career than Donald Trump ever has — but because he ruled against Trump, his loyalty to America is suspect

"I think it has to do perhaps with the fact that I'm very, very strong on the border, very, very strong at the border, and he has been extremely hostile to me," Trump told Fox News in February. That's the line he picked up in June to the Journal: that Curiel's heritage is a "conflict of interest," because his family is from Mexico and Trump wants to build a wall there.
Trump likes to say that if you don't have a border, you don't have a country. So if you are hostile to someone because he's strong on the border, what does that say about your feelings toward America?

Curiel's membership in a Latino lawyers association is proof he's a suspect individual

There are two actual pieces of evidence that Trump or Trump spokespeople have advanced to demonstrate Curiel's supposed bias. One is that he was appointed by President Barack Obama — something that is true of 320 of America's 860 authorized judges. The other is that he is a member of the La Raza Lawyers Association.
The La Raza Lawyers Association is a group of California Latinos in law trying to support and advance other Latinos in law. It's a pretty basic concept for a professional organization. It's only suspect if you think that any organization for a particular ethnic group is divisive, or (as members of the alt-right believe) that they're fighting for power and dominance in society and white people need to fight back.
In the eyes of the Trump campaign (or at least of spokesperson Katrina Pierson), though, the lawyers association is a radical group: "an organization that has been out there organizing anti-Trump protesters with the Mexican flags."

There is literally no evidence of anything like this. As best as I can figure, what's happening is that Pierson got confused by the phrase "La Raza"1 — which many racial conservatives use to stand in for the kind of radical Latino groups that are interested, they believe, in advancing their own ethnic interests at the expense of everyone else's.
In other words, this is the strongest actual argument that Curiel is biased against Donald Trump in a way that the other 37 percent of the federal bench appointed by Obama are not: that because he is a member of an organization that takes pride in Latino ethnicity, he is more interested in Latino interests than American ones. The only good Hispanic, in other words, is one who doesn't make too much noise about it.

This is literally about the patriotism of millions of Americans

Trump has been criticized for the "Mexican" thing because it's easy to pick on: It's verifiable wrong; people tend to understand that, on some level, it's being heard as a slur; and talking about people using "bad words" is often the easiest way to talk about racism. But it's a trap. At worst, it ends up clearing a path for people to express racist attitudes without getting criticized.
Focus on calling Judge Curiel "Mexican," and maybe — maybe — Donald Trump will stop doing it. (This is Trump, so it's arguably more likely that he'll call such cries PC culture and start doing it more often just to troll.) Even if you focus more broadly on the inappropriateness of Trump attacking a sitting federal judge for bias and calling for him to be investigated, you might, in a best-case scenario, get Trump to shut up about Curiel until the election.
But Gonzalo Curiel is not the only prominent person in America who's a member of an ethnic organization. If you think that being "strong on the border" and being a member of an organization with "La Raza" in the name are mutually exclusive, there are a lot of prominent Latinos whose loyalty to America comes under suspicion. And in a climate of rising Islamophobia where the Council on American-Islamic Relations is under suspicion in some circles as a terrorist fifth column, just think about what this says about the loyalty of Muslims.
And conversely, if you think that being a proud Latino isn't mutually exclusive with being a proud American, that's the principle you need to defend.