Immigration reform used to be an issue that split both parties: Pro-business Republicans faced off against cultural conservatives, while within the Democratic Party Latino advocates faced off against labor.
But over the past 10 years, immigration has become a partisan issue.
This change isn't just a shift in where politicians take certain policy questions. It's also a change in whether Americans think that immigrants, in general, are a good thing for America:
This is about culture first, and politics second
To a certain extent, polarization on immigration in Washington and polarization among voters reinforce each other.
In 2005, when comprehensive immigration reform was a key priority of President George W. Bush, Republicans and Democrats were about equally likely to think that immigrants strengthened America. Once Bush's successor, Barack Obama, started stressing the need for comprehensive immigration reform, though, Republicans were much less likely to look favorably on immigrants — according to Pew's findings, Republican attitudes changed precipitously between late 2009 and summer 2010.
But this isn't the whole story, because the debate in Washington over comprehensive immigration reform has always had a tenuous relationship to how Americans actually feel about immigrants.
Historically, even the "anti-amnesty" politicians who opposed comprehensive immigration reform stressed that immigrants were welcome in America as long as they came (and stayed) legally. For most Americans, though, the difference that matters isn't between legal and unauthorized immigrants — it's between immigrants they find likely to assimilate into "American culture" and those who (they believe) cannot.
Americans are much more ambivalent about immigrants, in general, than you might expect from listening to politicians talk about immigration — or than you might guess by looking at polling for various immigration reform proposals.
For many white Americans over the past couple of decades, that ambivalence has hardened into a constellation of stereotypes: associating "immigrant" with "illegal immigrant," "illegal immigrant" with "Latino immigrant," and "Latino immigrant" with "criminal."
This is the genius of Donald Trump's presidential campaign: His rhetoric homes in directly on the things that actually worry many Americans about immigrants, rather than using economic or legal arguments as a way to gesture toward cultural fears.
But as the chart shows, Trump wasn't just exploiting a sentiment among American voters — he's exploiting a sentiment among specifically Republican voters.
Anti-Latino sentiment has consolidated in the GOP
The changes shown in the Pew chart don't just reflect Democrats or Republicans changing their minds about whether immigrants are good for America. (In fact, most of the people changing their minds are embracing immigrants; overall, the most recent Pew poll found 59 percent of Americans agree that immigration strengthens the country, which is the highest level of support in 20 years.)
They reflect changes in who identifies as a Republican or a Democrat.
Over the past 20 years, the Democratic Party has gotten markedly more ethnically diverse...
...as white voters have increasingly identified with Republicans:
Not all of the white voters who have switched parties are motivated by anti-Latino sentiment. But the voters who are motivated by anti-Latino sentiment are particularly likely to have switched parties.
Before 2000, there was a correlation between negative feelings toward Latinos and identifying as strongly Republican. But that was just a side effect; how people felt toward black Americans was a much better predictor of how strongly they identified with the GOP.
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 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.
There's no indication that the trend has abated since 2008. Indeed, the Pew polling shows that sentiment toward immigrants among Republicans hit new lows in May 2015, with only 27 percent of Republicans saying immigrants strengthened America. That was just before Donald Trump launched his presidential campaign.
While Donald Trump didn't make Republicans wary of immigrants' effect on America, though, he does appear to have made the remaining skeptics in the Democratic Party embrace them.
In May 2015, on the eve of Trump's campaign launch, 62 percent of Democrats said that immigrants strengthened America. In March 2016, 78 percent said they did — a 16-percentage-point jump.
That's the biggest reason pro-immigrant sentiment is at a 20-year high: The Democrats who hadn't already embraced immigrants are doing so now.
If that holds, it will complete the last phase of the partisanization of immigration. Republican voters are already fairly united in their distrust of immigrants, and many Republican politicians are following their lead. Democratic politicians, meanwhile, are fairly united in their support of immigrants. And now, Democratic voters appear to be embracing their identity as the pro-immigrant party.