Donald Trump’s racist attacks against Gonazalo Curiel, the “Mexican” (American) judge handling two of the fraud lawsuits against “Trump University,” are not going over well with his fellow Republicans. Party leaders like Paul Ryan have denounced them—Ryan just called Trump’s line of reasoning “the textbook definition of a racist comment”—and figures like Lindsey Graham and Marco Rubio who seemed to have been making peace with Trump’s nomination have popped up to call him out. The usual excuses are being made, but even these seem to be grounded in a consensus that Trump crossed some sort of line.

But why these comments, after the many, many, many other ghastly things Trump has said about women, Muslims, undocumented immigrants, disabled people, and pretty much everyone else? I put the question to Slate’s crack politics staff. Here are some good guesses, and also my guesses, as to why the Republican establishment is not letting this one slide.

1. Trump is the nominee now. This is the first major controversy Trump has gotten himself into since he effectively clinched the Republican nomination. He’s now associated with the party in a way that he wasn’t when he made the other offensive comments. Republicans with their own reputations to protect no longer have any excuse not to respond to what he says.

2. This is exactly what Trump’s allies have been saying he won’t do anymore. Figures ranging from Ryan to Trump’s own campaign chairman have been claiming for weeks that Trump is going to grow up, as it were, now that we’re in general-election season. Top Republicans likely believe, and are justified in believing, that the kind of rhetoric Trump is directing toward Curiel is not going to help him get elected, and their public condemnations could be taken as signals to the candidate that he really needs to get his act together while there’s still time.

3. Trump’s comments were very clearly insulting to a very large electoral group. Muslims aren’t a major voting bloc; while Trump’s misogyny is readily apparent to anyone who has followed his career and campaign, he has generally avoided saying anything broadly offensive about women as a group; you can always try to get away with obnoxious comments about undocumented immigrants by claiming that you think legal immigrants are great. But calling a U.S. citizen like Curiel a “Mexican” sends a very clear message to every American of Latino descent: No matter what you do or how long your family has been here, people like Donald Trump will never consider you fully American. Meanwhile, there are roughly 27 million Latino voters in the U.S.

4. Curiel is a relatable and formidable foil. There’s a famous quote apocryphally attributed to Stalin: “A single death is a tragedy; a million deaths is a statistic.” It’s human nature to be more compelled by the story of one person than by generalizations about a group. Undecided voters suspecting that Trump is a little racist is one thing; undecided voters seeing Trump refer to an Indiana-born ex-prosecutor (who once had to live under protection on a naval base because he’d apparently been targeted for death by drug traffickers) as a “Mexican,” then insinuating that he sympathizes with border-crossing criminals, is another.

5. Trump is human-whistling rather than dog-whistling. The Republican Party’s electoral success relies on a coalition of white people that includes some who hold racist beliefs and some (many, even! Not all is lost) who do not. To avoid turning off nonracists while also keeping the racists aroused, Republican politicians use what are commonly referred to as dog whistles: statements that racist people would agree with that aren’t plainly racist because they’re about “culture” or, in the old days, “states’ rights,” rather than innate racial characteristics per se. (Here’s a good example of such a statement. Here’s another. And here is the best/worst one ever.) The GOP’s official position is ostensibly welcoming to people like Gonzalo Curiel, a child of immigrants who got an education, worked hard, and made good. The fact that Republican policies generally make it harder rather than easier for nonwhite immigrants and their children to get good educations and good jobs is, according to the party line, simply a matter of the market determining outcomes rather than a product of racial animus. But in Curiel’s case, Trump is breaking the party line and making that animus explicit.

So there you go! Those five items explain everything. Enjoy the rest of your day.

Read more Slate coverage of the 2016 campaign.