Immigration was a defining issue for President Trump in 2016. It’s less dominant in 2020, and it is motivating his critics as well as his supporters.

Border Patrol Agents, from left: Ben Taylor, Vinny Dulesky and Sergio Molina near the United States-Arizona border on Monday, Oct. 26, 2020.

    YUMA, Ariz. — It might be seen as a monument to the massive changes the Trump administration has brought to the country’s immigration system. Or it could be seen as a reason people are fighting fiercely to vote him out.

    Four years ago, President Trump promised voters he would build a big, beautiful wall. Whether the 30-foot, dark brown, steel fence towering over the sandy borderlands here is considered new, or beautiful, depends on one’s perspective. But it certainly is big.

    And it is a tangible example of how radically Mr. Trump has tried to make good on his promise to transform the immigration system, even if most of the changes have little to do with any physical barrier.

    In 2016, immigration was among the most defining aspects of Mr. Trump’s campaign. The idea of a border wall and the hard-line policies against asylum-seekers and undocumented migrants that it represented, helped sweep Mr. Trump into the White House. Four years later, the issue has taken a back seat, not only to the pandemic, but to protests over racial equity. The “build the wall” chants that reliably fired up the crowds at Trump rallies in 2016 has been replaced in large measure by rhetoric about “law and order.”

    The ‘Wall’ Is Still Motivating Voters. But This Time Is It Against Trump?

    A Biden-Harris banner is attached to a fence in front of the U.S.-Mexico Port of Entry in San Luis, Ariz. 

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    Since then, Americans’ attitudes have shifted significantly. In the years since Mr. Trump took office, voters have grown markedly more positive on immigration. In a June Pew Research Center poll, 28 percent of Americans said illegal immigration was a big problem, down from 43 percent last year. That included less than half of Republicans, compared to two-thirds the previous year. .

    Pew survey data in recent years has also shown that ICE stands as the country’s least popular agency. Pew regularly asks Americans for their opinion on a range of federal agencies, and almost every one tends to get a positive rating. But in a poll this spring, for the third year in a row, ICE was the only exception: Americans were divided in their opinion, 46 percent favorable and 45 percent unfavorable.

    And the partisan division is notable: Seventy-seven percent of Republicans gave ICE a positive rating, while just 28 percent of Democrats did. No other agency had nearly as stark a split.

    Signs calling for the re-election of the Trump-Pence ticket on farmland in Yuma, Ariz.  Credit…Gabriella Angotti-Jones for The New York TimesConstruction workers add a secondary wall to the United States-Mexico border near Gordon’s Well Imperial Sand Dunes park on Tuesday.Credit…Gabriella Angotti-Jones for The New York TimesA Border Patrol Agent at the border wall near Gordon’s Well Imperial Sand Dunes Park in California on Tuesday.Credit…Gabriella Angotti-Jones for The New York Times

    On the left, the fight against “kids in cages” — referring to the Trump administration’s 2018 policy of separating migrant children from their parents — has been something of a rallying cry for critics. Activists believe the policy has changed the 2020 landscape.

    “Family separation means that there was an issue everybody cared about, from a human rights perspective, and it gives us completely different political terrain, which is really significant,” said Jess Morales Rocketto, a Democratic activist and chair of We Belong Together, the National Domestic Workers Alliance immigration reform campaign. “We galvanized a ton of people. We have used people’s anger to make them understand that the way we treat our most vulnerable is related to who they elect.”

    In Tucson, Mayor Romero knows immigration is a deeply personal issue for many voters here. At a rally with Senator Kamala Harris, the Democratic vice-presidential candidate, in Tuscon this past week, Ms. Romero referred to the border barrier as “the wall of hate,” and has repeatedly criticized the environmental impact of the new construction and deaths along the border. Still, Ms. Romero is optimistic about Democrats’ chances in the state, where Latinos are expected to make up at least 25 percent of all voters for the first time.

    For supporters of Mr. Trump, however, the administration’s immigration policies have created a kind of fierce loyalty. The union for the Customs and Border Protection gave Mr. Trump its first presidential endorsement in 2016 and did so again this year, convinced that the president has done what others have shied away from.

    “The morale is higher, it is much higher because border patrol agents feel like they have an administration that actually cares,” said Brandon Judd, the president of the union and an ally of Mr. Miller. “At the end of the day, border patrol agents want to feel like they are productive; they want to feel like they’ve made a difference.”

    Senator Kamala Harris concludes her speech during a drive-in rally at Pima Community College’s West campus in Tucson.Credit…Gabriella Angotti-Jones for The New York Times

    For the most part, Mr. Biden and other Democrats have defined their own policies by what they are not — promising an end to family separations, an end to draconian asylum policies, an end to the travel ban — rather than what they are.

    After news broke earlier this month that several hundred migrant children who had been separated from their parents at the border had not been reunited with their parents, the topic of family separations surfaced during the last presidential debate. Mr. Biden has said he would end the practice and has vowed to work to reunite families.

    But deportations also skyrocketed under the Obama administration, and immigration activists eventually dubbed the former president the “deporter in chief.” And while Mr. Biden has tried to distance himself from those policies, he is certain to face pressure to address the estimated 11 million undocumented immigrants now living in the United States should he win the presidency.

    “Frankly speaking, immigrants are not a priority for the Democrats,” said Pablo Alvarado, who has helped campaign for Mr. Biden in several battleground states and is the executive director of the National Day Laborer Organizing Network “We’re more of a priority for the Republicans for the wrong reasons — to attack us, to stigmatize us, to persecute us.”

    Still, he added: “This time around, we are not going to be so polite, because otherwise what happens is exactly what happened the last time — they decide to do many other things first. We’re going to have to be a little more aggressive this time. At this point, what other choice do we have?”

    Giovanni Russonello and Hank Stephenson contributed reporting.