When Donald Trump said that "murderers" and "rapists" were coming over the US/Mexico border during his campaign launch, it was just one in a barrage of outrageous lines. It only became the centerpiece of his presidential campaign — the message he's ridden all the way to the Republican nomination — when he realized that was the outrageous line that hit a nerve.

He embraced it because liberals hated it: When Robert Costa of the Washington Post asked him in mid-July how he settled on immigration as a central campaign issue, Trump replied "they gave it to me" — "they" being the media outlets and Latinos who started raising a fuss. But he's stuck with it, and it's helped win him the nomination, because many conservatives love it.

When Trump launched his campaign, his real constituency was the media — not any group of voters per se. His goal was to shock the media into paying attention to him. But as it happens, he stumbled into something even more valuable: an actual constituency.

The message he found won a real and intense emotional response from some group of Americans — powering him to the top of the Republican primary race, and now, presumptively, the nomination.

Trump likes to say that no one was talking about immigration before he entered the race. That isn't true. But the way he talked about it resonated much more deeply than anything his rivals could muster.

Americans' feelings about immigration are tricky: Some people are categorically anti-immigrant, others care a great deal about the distinction between legal and illegal immigrants, and a large share judge immigrants on how close they are to an imagined "traditional American culture."

Before Trump, Republicans talked about immigration in terms of policy. They found messages that were acceptable to their conservative base, but struggled to find messages that excite them.

Trump has succeeded where they've failed: He's found a message that gets to the core of why so many conservatives are ambivalent or hostile toward immigrants. Here's why.

1) Millions of Americans are opposed to all immigration

Because immigration is such a complicated political and cultural issue, the best insights into how Americans really feel about immigrants — and why they feel that way — have come from experimental political science.

In one type of study, researchers mix together different hypothetical immigrant traits — a legal Mexican laborer, an unauthorized European student — and ask people the same questions about these hypothetical individuals. That's allowed researchers to disaggregate feelings about immigrants from feelings about whatever ethnic group Americans picture when they think of the word "immigrant."

One of these studies, published in 2014, straightforwardly asked subjects to determine whether they'd allow a hypothetical immigrant to come to (or stay in) the US legally. The results offer a clue as to how many Americans are genuinely anti-immigration — or anti-illegal immigration — across the board.

Twelve percent of respondents were categorically opposed to all immigration — legal and illegal. That seems small, but it represents millions of Americans (who, as we'll get to later, are likely to identify with the Republican Party).

Most Republican candidates in recent years haven't made an effort to appeal to those voters: The standard Republican line on the issue is that they welcome legal immigrants but reject illegal ones.

Trump has occasionally drawn the same distinction. But he doesn't spend nearly as much effort praising legal immigrants as he does attacking undesirable ones. And his immigration platform — written with the consultation of Sen. Jeff Sessions, Congress' most vocal opponent of both legal and unauthorized immigration — is pretty darn harsh on legal immigrants, too.

2) The Americans who reject all illegal immigrants are the ones who really care about paying parking tickets

Another, larger group of Americans (20 percent or more) aren't opposed to all legal immigration, but are categorically opposed to illegal immigration or giving legal status to unauthorized immigrants. In the 2014 study, those respondents were no more sympathetic to European immigrants without papers than Guatemalan ones — none shall pass. Taken with the 12 percent who reject both legal and illegal immigrants, we're talking about a third of Americans who reject "illegal immigration" out of hand.

This isn't necessarily due to racism or xenophobia — after all, these people wouldn't let in even a hypothetical white or educated immigrant. Study author Dr. Matthew Wright of American University, whom I spoke to in 2015, pointed out another possible explanation: People who reject all illegal immigrants but not legal ones are simply, dispositionally, more likely to see the rule of law and honesty as very important traits.

"If we ask people other questions about, 'How important is it to pay your taxes? or 'How important is it to pay a parking ticket?' — all these kinds of questions that sort of loosely tap into the idea of being a law-abiding member of society — that becomes very important in predicting whether or not these people are going to accept illegal immigrants," says Wright.

These are exactly the people Republicans (including, occasionally, Trump) are playing to when they say that they like immigration, just not illegal immigration. But while the rule-of-law voters are thoroughly in line with the position of most Republicans, they're particularly well-primed to like Trump's message that immigrants are violent criminals: After all, they assume that immigrants here without papers have already demonstrated that they don't respect the law. (This can get very confusing when people talk about policy — just check out the extremely confused reports claiming that unauthorized immigrants commit a majority of crimes in America, when what's really going on is that a majority of cases in federal court are for violating immigration laws.)

But because rule-of-law voters already have a voice within the GOP, Trump's ability to speak to them wasn't what powered his rise. Where Trump's real demonstrated brilliance lies is in talking to the — much larger — group of white Americans whose opinions about immigrants are ambivalent for reasons that have nothing to do with legality.

3) For a plurality of Americans, what matters isn't legal status — it's whether immigrants are "like them"

For many white Americans — the Republican Party's most important constituency, in both the primaries and the general election — immigration isn't as simple as legal versus illegal. Their primary concern is preserving American culture.

It's not that these Americans care less about immigration than the people who are categorically opposed — to the contrary, these are often the people who are most concerned about who's coming into the US. It's just that their concerns don't line up with typical policy messages.

Surprisingly, Wright and his co-authors found that it wasn't common for Americans to care some about an immigrant's legal status. Either they accepted (or rejected) every single hypothetical unauthorized immigrant, or they accepted unauthorized immigrants about as often as legal immigrants — which is to say, they didn't care about legal status at all.

Most of these weren't open-border supporters who accepted every immigrant they were asked about. They were just looking at other factors: employment, education, religion, and national origin. An unauthorized Christian immigrant fared better than a legal Muslim one. An unauthorized immigrant from France fared better than a legal immigrant from Mexico, but an unauthorized immigrant from Mexico fared better than a legal immigrant from Somalia.

In other words, says Wright, this group of people cares whether an immigrant will contribute to the community economically, and whether she will assimilate culturally.

That's not all that surprising. After all, polls show Americans are surprisingly ambivalent about whether immigration is a good thing for American culture. Agreement that immigrants strengthen the US rather than burdening it keeps rising, but very gradually:

Wright's study found that when looking at particular immigrants, economic factors mattered alongside cultural ones. But other studies have shown that American anxiety about immigrants' cultural impact is much more consistent than economic concerns.

As one study put it, "Evidence about the role of economic concerns in opposition to immigration [...] has been inconsistent. On the other hand, symbolic attitudes such as group identities turn up as powerful in study after study."

And Latino immigrants generate more anxiety than immigrants from other regions. One study found that Americans didn't perceive Latinos as more of a threat than immigrants from other regions, but they were more anxious about them. Another found that when asked about hypothetical immigrants breaking laws or — especially — cultural norms (like stepping on an American flag), Americans were much more offended when the hypothetical immigrant was Latino than when he was European.

Unfortunately, we don't know how many white Americans are seriously anxious about the cultural threat immigrants pose. We do know, however, that when anxieties about immigration are higher, people become more likely to identify with the GOP. A 2015 book called White Backlash by UC Berkeley political scientists Zoltan Hajnal and Marisa Abrajano concluded that "greater opposition to increased immigration nationwide" in one quarter was linked to an increase in partisan identification with the GOP in the next quarter.

Even the non-Trump Republicans who took the hardest line on immigration policy — Rick Santorum and (sometimes) Scott Walker — addressed it in economic terms: They're worried about American jobs. But Americans are less worried about American jobs than about American culture.

Trump, on the other hand, has repeatedly accused the government of Mexico (and possibly other countries) of deliberately sending its worst people to immigrate to the United States. That's much closer to the heart of American anxiety.

4) Americans conflate "immigrant" with "illegal immigrant," and "illegal immigrant" with "Mexican immigrant"

One study asked respondents how they feel about "immigrants" in general, or in various scenarios, and compared that with how they respond to "legal immigrants," "illegal immigrants," "European immigrants," etc.

They found that the baseline favorability for "immigrants" in general was generally closer to how Americans felt about "illegal immigrants" than legal immigrants — as if people thought of immigrants as illegal by default, and only remembered about legal immigrants when prompted.

And Americans do the same thing with nationality — their feelings about "immigrants" are similar to their feelings toward Latin American immigrants, as opposed to European or Asian ones. A separate study found that Americans answered identically when asked a question about "immigrants" and the same question about "Mexicans."

Abrajano and Hajnal write in White Backlash, "Americans tend to reserve their most negative sentiments for so-called illegal immigrants, but when asked about immigrants as a whole, Mexican Americans, or even Latinos, the answers tend not to differ all that much."

Or look at the phenomenon another way: Americans vastly overestimate how many immigrants there are in America, and how many of them are here illegally. But their estimates of unauthorized immigrants are strikingly similar to the US's total Latino population. They may not actually believe that every Latino is an illegal immigrant, but they appear to be taking cues about one from the other.

The upshot: "When members of the US public think about immigration," Abrajano and Hajnal write, "they are likely to have a picture of a Latino or Mexican American coupled [with] an impression that they are in this country without legal documentation. In the minds of many white Americans, these different categories simply blur together." And they "blur" in the direction of the immigrants they view least favorably.

That's why Donald Trump can get away with focusing on illegal immigration at some times and carefully letting it slide other times: His audience knows whom he's talking about.

5) In the 21st century, anti-Latino and anti-immigrant sentiment have become important even independently of plain old anti-black racism

It's not surprising that these associations exist. What's surprising is how recent they are as a development. It's only in the last quarter-century that "immigrant" has become identified distinctly with Latinos in the American mind.

As recently as the early 1990s, Americans who supported restrictive immigration policies were more likely to feel negatively toward nonwhites in general — but their feelings toward individual groups of nonwhites didn't matter as much. Beginning as early as 1994, though, and incontrovertibly by the 21st century, how Americans felt about Latinos in particular became a proxy for how they felt about immigration.

Their feelings about Asian Americans — or even black Americans, who have historically borne the brunt of American racism — weren't as relevant. People who disliked immigration disliked Latinos in particular.

You can see the same effect on partisan identification. Before 2000, according to the authors of White Backlash, there was a correlation between negative feelings toward Latinos and identifying as strongly Republican — but the real explanatory variable was how people felt toward black Americans.

In the 21st century, the two have diverged. All else being equal — even sentiment toward African Americans — a white American in 2008 who felt negatively toward Latinos was likely to be slightly more strongly Republican (one-third of a point on a seven-point scale from strong Republican to strong Democrat) than someone who felt positively toward them.

6) Media coverage of immigrants has traditionally focused on crime

The immigrant population is more diverse today than it was in 2000. This year, by one estimate, China surpassed Mexico as an immigrant-sending country. So how is it that as fewer immigrants (comparatively) are Latino, Latinos have become the face of immigration?

One answer is demographic — Latinos have been moving into areas where they traditionally weren't, including conservative white areas like the Deep South and Midwest. (Conversely, older whites have moved to places in the Sun Belt where Latinos have lived for generations.)

But another answer is that the media talks a lot more about Latino immigrants than it does about other kinds of immigrants. And especially during the Bush era, when the media talked about immigrants they often talked about crime.

In 2006, two-thirds of news coverage of immigrants focused on crime and terrorism. (As immigration reform has become a perennial political issue throughout the Obama administration, politics has become a more typical frame than crime — but there's no reason that can't change.) And the more the media focuses on immigrant crime, the more likely anxious Americans have been to identify with the Republican Party.

In polls, Americans as a whole are, if anything, even less anxious about immigrant crime than about immigrants' effect on the economy. In fact, when asked directly in a 2015 Washington Post/ABC News poll, an overwhelming majority of Americans — and even a majority of Trump supporters — didn't actually believe that most Mexican immigrants are "undesirable people like criminals."

But when crime is tied to the anxiety about particular kinds of immigrants, and anxiety about losing the American way of life, it becomes much more powerful. The authors of White Backlash call this the immigrant "threat narrative."

Trump talks about the group of immigrants that Americans distrust most and feel most anxious about — in a way that focuses attention on the threat they pose to the American way of life. And while he claims that he's brought up the issue of immigration all on his own, he's tapping into a sentiment that's been well developed by media coverage and cultural associations.

7) This is culture, not policy

The "white backlash" population has become concentrated in the Republican Party over the last several election cycles. For a certain group of Republicans, that anxiety is an important difference between them and "mainstream" America.

This isn't saying anything new: The resurgence of populist conservatism with the Tea Party in 2010 has always been motivated in large part by a need to "take back America," a feeling that the country conservatives had grown up in was being threatened (or had already been replaced) by a more diverse and foreign version.

But that's fundamentally not a policy concern. It's a cultural concern that is often expressed through policy, because policy is an easy way to talk about it. And that's especially true for immigration policy. It's hard to oppose a diversifying America; it's easy to oppose immigration reform.

This was the real difference between Trump and the Republicans he beat for the nomination. Other Republicans continued to talk about immigration policy — what they would do with the millions of unauthorized immigrants currently in the US, whether America needs more high-skilled foreign workers or fewer. That was one degree removed from why the people they were talking to cared about the issue. They care about immigration because they are worried about the threat immigrants pose.

Trump is right there with them. His policy ideas are absolutely ludicrous. But he's not really talking about the solutions. He's talking about the problem. And that means he's expressing the feelings that many Republicans don't hear from other politicians.

This is an updated version of an article originally published on July 29, 2015.

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