President Trump walks to Marine One on the 24th day of the partial government shutdown, the longest in U.S. history. (Jabin Botsford/The Washington Post)

This past weekend, the partial shutdown affecting a quarter of the federal government became the longest on record. Most Americans do not support the shutdown: In a recent NPR-Ipsos poll, 71 percent said Congress should pass a bill to reopen the government while budget talks continue. And more Americans blame President Trump: In a new Washington Post poll, 53 percent said Trump and congressional Republicans were responsible for the shutdown, while only 29 percent blamed congressional Democrats.

Yet Trump appears unwilling to change course. Last week, he walked out of a negotiating session with congressional Democrats when House Speaker Nancy Pelosi (D-Calif.) told him that they would not fund a border wall. Meanwhile, Senate Majority Leader Mitch McConnell (R-Ky.) won’t consider House-approved bills that would reopen the government because Trump won’t support them.

Why is there such difficulty finding a path forward? The roots of the shutdown can be traced to the fraught issue of immigration. A new GW Politics Poll shows just how hard it will be to reopen the government if that depends on a bipartisan immigration compromise.

1. Republicans identified immigration as the most important issue for Congress this year.

The GW Politics Poll interviewed 1,920 registered voters in December, before the shutdown, about what issues should be priorities for Congress this year. For Republicans, immigration was the top issue: Eighty percent said “dealing with the issue of immigration” should be a priority for Congress this year. Immigration was followed closely by “defending the country from future terrorist attacks” (79 percent said it was a priority) — an issue the Trump administration has misleadingly tried to link to border security — and by “strengthening the nation’s economy” (78 percent said it was a priority).

By contrast, only 41 percent of Democrats said immigration was a priority.

Thus, it’s clear that a president who prioritizes support from the GOP base has an incentive to focus on immigration.

2. Republicans are deeply concerned about immigrants entering the country.

During the 2018 campaign, Trump focused on a caravan of Central American migrants headed to the U.S. border to seek asylum. The GW Politics Poll asked respondents how concerned they were that some of these migrants might gain entry to the United States.

There was a remarkable partisan divide on this question: Sixty percent of Republicans were very concerned, and 21 percent were somewhat concerned. But only 19 percent of Democrats were somewhat or very concerned, and 47 percent were not at all concerned. The same partisan divide emerged in The Post poll’s question about whether the situation at the border was a “crisis.”

It’s no surprise, then, that Republicans were far likelier than Democrats to support sending U.S. troops to the border in response to these migrants: Eighty-five percent of Republicans favored this, but only 14 percent of Democrats did.

3. Partisan polarization on immigration issues is enormous.

Indeed, Democrats and Republicans are deeply divided not only on the caravan but also on nearly every issue related to immigration that we asked about in this poll.

For example, majorities of Republicans supported temporarily banning Muslims from other countries (74 percent); separating undocumented parents from their children at border crossings to discourage others from coming (58 percent); and making it harder to immigrate to the United States legally (56 percent). Only about 1 in 10 Democrats supports any of these things. And although this poll did not ask about a border wall, Democrats and Republicans differ on that, too, largely because of declining support among Democrats after Trump began advocating for it.

4. Views of immigration are fairly entrenched.

The GW Politics Poll is unique in that we tracked the same group of voters over 2018, interviewing them in May, August, October and December. Across these months, and despite headline events involving immigration, there was little movement in opinion. Over a longer period, any movement has been only toward more polarization. Republicans have become more supportive of a border wall in The Post polling. In other polls, Democrats have become more favorable to immigration.

What this means for an immigration compromise

These differences may give leaders little reason to think their supporters want a compromise on immigration.

Consider the much-discussed compromise of border wall funding in exchange for legal residency and work permits for undocumented immigrants brought here as children, often called “dreamers.” This compromise has foundered before, including when Trump rejected it in January 2018. This poll helps explain why: Nearly half of Republican voters oppose helping dreamers in this fashion.

Moreover, the 2018 election left the House GOP with fewer centrists potentially willing to compromise on the issue anyway. To get Republicans on board with a compromise, they’ll need some cover from Trump that he seems unwilling to provide.

There’s also a big group of Democratic voters who don’t want compromise. As Scott Clement and Dan Balz wrote, The Post poll found that “42 percent of Democrats who oppose the border wall say congressional Democrats should refuse to budge even if it extends the shutdown.” How would this large plurality of Democrats respond to a deal that builds more of a wall? It’s not clear that either Democratic or Republican voters want a compromise.

The shutdown is thus one more consequence of the increasingly divisive politics of immigration. Only 12 years ago, Democrats and Republicans had very similar views of whether to increase or decrease immigration. Now they are miles apart. So any government decision that depends on an immigration compromise will be difficult to achieve — even something as basic as keeping the government open.

Kimberly Gross is an associate professor of media and public affairs at George Washington University.