We’re facing the third government shutdown in less than a year this Friday thanks, in part, to a fight over immigration policy. President Trump wants $5 billion for a border wall — an amount that is unlikely to make it through the Senate. Back in January, a disagreement over the Deferred Action for Childhood Arrivals program caused a partial government shutdown. So it’s worth taking a step back and asking: Why is immigration such a stumbling block?

After all, it wasn’t always like this. Conservatives once backed more liberal immigration policies, and liberals have at times backed more restrictionist ones. In 1986, for example, Ronald Reagan signed a law that granted amnesty to nearly 3 million undocumented immigrants. Reagan and George H.W. Bush both used their executive powers to declare that children of undocumented immigrants affected by the Reagan-era law could not be deported. In 2006, Barack Obama and Hillary Clinton, who were both then senators, voted for 700 miles of additional fencing on the U.S.-Mexico border as part of a provision to satisfy conservatives concerned about a rise in illegal immigration.

But over the past couple of decades — as the number of unauthorized immigrants in the U.S. rose steeply and then began to decline — immigration policy has come to symbolize the two parties’ broader values and electoral coalitions. The battle over immigration policy is about way more than just immigration, in other words, in the same way that the tensions between the two parties on health policy reflect deeper fault lines. The politics of immigration today are notably more divided and partisan than they were 10 or 20 years ago, and there are a few reasons why.

First, there are the party coalitions. Compared to the mid-2000s, the Democratic Party of today includes fewer non-Hispanic white voters: 67 percent people who are or lean toward being Democrats were non-Hispanic whites in 2007, but that number had dropped to 59 percent in 2017, according to the Pew Research Center. Forty percent of self-identified Democrats are now nonwhite. Republicans too have grown more racially diverse, but only barely, and they are still overwhelmingly white: 88 percent in 2007, compared to 83 percent in 2017. About 12 percent of Democrats are Hispanic, roughly double the percentage of Republicans who are of Hispanic descent.

So the Democrats have a huge bloc of people in their party who have racial, ethnic and cultural ties to America’s most recent immigrants, who are largely Asian- and Latino-American. And while “minorities” and “people of color” are fraught terms that often ignore differences both between and within racial and ethnic groups, the Democrats are essentially now the home party for Americans who might feel that U.S. society treats them as “other.”

Secondly, while both parties have undergone ideological shifts, Democrats have shifted more dramatically. Pollsters ask a variety of questions to measure public opinion on immigration, but they all show the same thing: Democrats have become far more pro-immigration in recent years.

According to Pew, in 2006, 37 percent of Democrats said that legal immigration to the U.S. should be decreased, compared to 20 percent who said it should increase. Pew found a huge reversal in those numbers earlier this year: 40 percent of Democrats back higher immigration levels, compared to 16 percent who want them lowered. According to Gallup, 85 percent of Democrats now feel immigration is a “good thing” for America, compared to 69 percent who said the same in 2006. Republicans haven’t actually become more anti-immigration, according to Pew and Gallup. But, per Pew, there are more Republicans who want immigration decreased (33 percent) than who want it increased (22 percent).

As a result, the gap between the parties on questions about immigration has become a chasm:

And immigration is indicative of a broader shift: Democratic voters have grown more liberal on issues of race, gender and identity generally. That includes white Democrats.

The voters are not alone. Elites in each party have moved toward the ideological poles on immigration policy. Liberal-leaning activists and Democratic politicians argue that policies like the wall aren’t just bad or ineffective, they are immoral and racist. Trump and other conservatives have suggested that more immigration could both hurt the U.S. economy and lead to more crime.

Let me avoid making this a both-sides story: For the most part, Democrats are more aligned with overall public opinion on immigration. The majority of voters want undocumented young people who were brought to the U.S. as children to be protected from deportation, and Democrats’ demand for that provision that led to last winter’s shutdown. Likewise, most voters don’t support a border wall, but Trump is driving toward a shutdown in pursuit of a wall, an idea that many congressional Republicans are fairly lukewarm about.

That said, America did elect a president (in 2016) and a Senate majority (in 2016 and 2018) who belong to the party that is generally less supportive of immigration, so either there is some appetite for a middle ground or immigration is not a deal-breaker issue for many Americans. Either way, it would be logical for the two sides to find a compromise. But the shifts the parties have undergone in the last 10 or so years make such a compromise hard to execute. Democratic leaders can’t easily sign on to any funding for a wall that their base thinks is a physical monument to racism, particularly since the top Democratic leaders are white but much of the party base is not. Trump can’t easily give up on the wall, since he basically campaigned on the idea that America needs a wall to remain a great nation.

So we’re already at two shutdowns involving immigration policy in the Trump era — and I would not rule out a few more.