Before Donald Trump’s movement was about anything else, it was about this: the fear that unauthorized immigrants would victimize — even kill — Americans.

That’s how he first went from a joke celebrity candidate to a man who could fill stadiums for campaign rallies. It’s how he won the Republican nomination. And it’s how he opened his Republican National Convention.

It made for the most compelling speeches of the first night of Trump’s convention, as Mary Ann Mendoza, Sabine Durden, and Jamiel Shaw — parents of Americans killed by unauthorized immigrants — spoke one after another.

The speeches were devastating. "One minute I was hearing his voice, ‘Be right home, old man!’" Shaw said.

They were also clarifying. They were a welcome reminder of exactly why Trump succeeded where so many other politicians have failed.

"Only Trump mentions Americans killed by illegals," said Shaw. "Trump will put America first."

Despite the months-long inter-pundit argument about whether, and to what extent, Trump supporters are motivated by "economic anxiety," politicians have tried to power anti-immigration movements on economic anxiety before. It didn’t work. Trump powered his movement on anxiety about the immigrant threat.

How Donald Trump stumbled into an anti-immigration campaign

Let’s go back to June 2015, when Donald Trump descended the infamous staircase and formally launched his campaign for president.

Trump’s launch speech was all over the place. He talked about Obamacare, about China, about immigration, about Iran. Trump’s speeches may seem rambling now, but that’s deceptive: He’s learned which lines the audience responds best to, and he makes a point of building up to them. When he launched his presidential campaign, Trump didn’t appear to know where the applause lines were.

He soon found out.

Liberal groups and media outlets seized on Trump’s assertion that Mexico was sending "murderers" and "rapists" into the US illegally. That was good news for Trump: He stayed in the news. But they weren’t the only ones.

Trump’s remarks also caught the attention of a group of people who’d been arguing for years that unauthorized immigrants were a threat to public safety and national security.

That group included people like Mendoza, Durden, and Shaw. Shaw appeared with Trump at Trump’s first campaign mega rally in Arizona. Durden, for her part, said on Monday, "I have been talking about illegal immigration since 2012, and no one listened until Donald Trump. Donald Trump is not only my hero, he’s my lifesaver."

It included border patrol agents (and their union) who felt the Obama administration was undermining the safety of American citizens. One local union offered to give Trump a tour of the border early in his campaign (before the national union, an AFL-CIO affiliate, squashed that plan). The national union endorsed Trump — the first time they’ve ever endorsed a presidential candidate.

Despite early reporting, border patrol agents didn’t speak at Trump’s convention. But family members of Border Patrol agent Brian Terry, who was killed by guns later traced to the gunwalking scandal known as "Fast and Furious," gave their speech live from the US/Mexico border — and turned Terry’s death into an indictment not of the Fast and Furious operation but of the Obama administration’s refusal to secure the border.

It included Alabama Sen. Jeff Sessions (who also spoke tonight), who just a few years ago had been an outlier in a Republican Senate delegation that generally favored immigration reform — and who now became not only an early endorser of Trump but also a key adviser who helped write Trump’s immigration platform and select a would-be vice president.

Trump’s embrace of people like this — and their embrace of him in return — explains why his immigration policy has always been an order of magnitude more sophisticated than his policies in other areas (if no less terrifying for that reason). But Donald Trump didn’t just stumble into a community of engaged activists. He stumbled into a large, underserved constituency within the Republican Party itself — a constituency that had been underserved because politicians weren’t talking to them about immigration in the ways that reflected what they were actually worried about.

Trump has always been the "law and order" candidate, even before he used the term himself

Politicians often talk about opposition to unauthorized immigration (or to legal immigration) as an economic issue. It’s not enough to get them the nomination of a major party for president of the United States.

That’s because, at root — as both polls and experiments have demonstrated — Americans who are anxious about immigration aren’t anxious that immigrants will take their jobs. They’re anxious that immigrants will change the character of their country, replacing traditional "American values" with something else.

When Donald Trump talked about Mexico "not sending their best people" — when he said, "If you don’t have a border, you don’t have a country" — that is what he tapped into.

That’s not exactly the same as saying people fear immigrants because they’re worried about crime. In fact, Americans as a whole are, if anything, even less anxious about immigrant crime than about immigrants' effect on the economy. But when crime is tied to the anxiety about particular kinds of immigrants, and anxiety about losing the American way of life — what political scientists Marisa Abrajano and Zoltan L. Hajnal call the immigrant "threat narrative" — it becomes much more powerful.

When Donald Trump trots out the relatives of people who were killed by unauthorized immigrants, he’s underlining that association. "I had my son’s life stolen from me by a man who was three times the legal limit drunk, was high on meth, and drove for over 25 miles the wrong way. And he had no business being in this country," Mendoza said Monday night.

For Trump and his followers, it’s not surprising that "illegal immigrants" commit crimes — "illegal" is right there in the name. (On the campaign trail, Trump has taken to reciting the lyrics of an old R&B song that ends, "You knew damn well I was a snake before you took me in.") They don’t respect American values, and that is why they are a threat.

Trump has recently taken to calling himself the "law and order" candidate — deliberately using a phrase Richard Nixon used in 1968, when he too tapped into white Americans’ fears of cultural instability through racially coded language. By claiming the "law and order" mantle, though, Trump was just making the subtext text. The ability to tap into white Americans’ cultural anxieties about nonwhites destroying America is something he’s been doing, to great success, all along.

"Are we adding voters, or swapping them?"

To state the obvious: The general election is not the Republican primary. It’s not always clear that Donald Trump understands he has to make at least some changes to win the general. The Republican Party around him, for the most part, does.

In particular, you can’t win a general election with only white voters. And while Trump’s brand of white identity politics has excited voters who weren’t hardcore Republican Party activists before, it’s also driven nonwhite voters even more deeply into the arms of the Democratic Party than they were in 2012.

Trump is quick to say he loves the Hispanics. His supporters are quick to point out that he only said some Mexican immigrants were rapists and murderers — and said others were good people. Those justifications do not appear to be playing well to Latino voters. Latinos are used to people assuming all Latinos are immigrants; assuming all immigrants are unauthorized immigrants; and assuming all unauthorized immigrants are criminals. Many of them are aware that the Obama administration, under the guise of targeting "criminal aliens," deported a record number of unauthorized immigrants who posed no public safety threat.

They’re certainly aware that the rise of Trump, at least by all appearances, has emboldened a kind of hate crimes of everyday life: immigrants and Latinos being taunted at school basketball games or discriminated against at drugstore pharmacies.

And even though Trump has been much less interested in dog-whistling about African Americans than he has in dog-whistling about Latinos and Muslim Americans (who he says are all unassimilated at best and potential terrorists at worst), black voters know dog-whistling when they see it, and they’re well aware that the people Trump has activated don’t love black people either.

Certain Republicans understand this is a problem. Paul Ryan asked rhetorically on Monday, "Are we adding voters or swapping them?" In reality, "swapping" might be optimistic — even if the GOP is adding older white voters, those voters will be outlived by the younger nonwhite ones they lose.

As the Republicans raised the curtain on their convention, many of them at least acknowledged that Trump 2016 has a problem with nonwhite voters — and implied he was working to fix it. Republican National Committee Chair Reince Priebus declared on Sunday that Trump would go on a "Hispanic engagement tour" shortly after the convention ended. On Monday, the campaign announced the hire of an African-American outreach director — sure, it was former Apprentice contestant Omarosa Manigault, but still.

The Latino Partnership for Conservative Principles, which had denounced Trump during the primary, issued a statement on Monday endorsing him. "We are not enamored with Donald Trump’s bombastic rhetoric and personality," the "conservative leaders" signing the statement wrote. "But at least we know that Donald Trump won’t divide the country and fuel the flames of racial confrontation by accusing the police and the entire criminal justice system of being racist."

That’s true. Donald Trump has never picked that particular way to "fuel the flames of racial confrontation." He does it in other ways. And his "Make America Safe Again" program proved he’s not yet willing to retire the dog whistles that brought him this far.