MEXICO CITY — When Ken Salazar was in the U.S. Senate, he believed the United States was nearing a solution to its long political impasse over immigration. He had helped craft a piece of bipartisan legislation to address border security and a path to legal status for undocumented immigrants.

That was 14 years ago. The legislation never passed. Now the Colorado Democrat, who arrived in Mexico last week to take up his post as U.S. ambassador, finds himself at the center of a more polarized moment on migration — one he’ll tackle from the other side of the border.

“The challenges have not gone away,” Salazar, who also served as the secretary of the interior in the Obama administration, told The Washington Post this week.

Immigration tops the list of challenges confronting the United States and Mexico as the Biden administration attempts a “reset” in relations. Former president Donald Trump vilified the country during his first election campaign, made good on a pledge to renegotiate NAFTA and alarmed Mexicans by threatening to impose steep tariffs if the government didn’t crack down on migration.

Salazar said the Biden administration is trying to broaden the agenda beyond Trump’s focus on migration — he noted a recent meeting of Cabinet members from both sides to discuss economic issues, and an upcoming session on security cooperation.

“We’re in the middle of a reset,” he said.

Still, as Republicans attack Biden over a surge in migration, the issue remains the most urgent between the neighbors.

U.S. immigration agents have made more than 1.2 million apprehensions along the southern border in the last 12 months, the most in more than a decade. For 11 million undocumented immigrants in the United States, there’s still no path to legal status.

Migrants seeking asylum in the United States rest in a makeshift tent near the International Bridge as they wait Thursday to be processed in Del Rio, Texas. (Go Nakamura/Reuters)

In an attempt to curb the flow of migrants, the United States has once again enlisted Mexico’s help: The Biden administration has asked the country to detain migrants on their way north. It has also sent planeloads of Central Americans to southern Mexico so they’ll have a more difficult time returning to the border.

Mexico has begun to bristle against the seemingly endless U.S. requests that it do more on enforcement.

Salazar, one of the first Mexican American senators and cabinet members in U.S. history, arrives at his new post as the Biden administration attempts to project both a humanitarian sensibility on border security while also deterring immigration. By some measures, it is doing neither, adopting Trump-era curbs on asylum while failing to reduce the flow of migrants.

The beginning of Salazar’s term here, some hope, could help refocus the issue. He is Biden’s first ambassador to be confirmed by the Senate.

Salazar once found an ally on immigration reform in President George W. Bush. Now, he is attempting to nurture a similar relationship with Mexican President Andrés Manuel López Obrador. His first trip in Mexico this week will be to López Obrador’s home state of Tabasco, where Salazar plans to visit the country’s porous border with Guatemala.

Officials here have welcomed Biden’s efforts to expand the bilateral agenda and take on Mexican priorities such as the southward flow of guns over the border, a factor in the country’s extraordinary violence.

“Definitely it’s a new era, with a very broad dialogue,” said Roberto Velasco, chief North America officer at the Foreign Ministry.

Nonetheless, Mexican officials have grown frustrated with what they consider insufficient U.S. action to address the factors that lead people to migrate, particularly from Honduras, Guatemala, El Salvador and southern Mexico. Since the Obama administration, the United States has leaned on Mexican security officials to detain migrants, even as human rights groups have raised concerns about Mexican enforcement schemes.

This year, Mexican immigration agents have stopped 147,000 irregular migrants, according to the country’s migration agency.

Mexican officials believe it is partially a broken U.S. immigration system that encourages migrants to come to the border. Analysts have noted their growing frustration.

“There’s been a tendency in the U.S. government to lean on Mexico to do border enforcement so they don’t have to,” said Andrew Selee, the president of the Migration Policy Institute. “Salazar is going to have to be sensitive to Mexican concerns about doing more on enforcement while also building real legal pathways for migration.”

Mexican officials argue that a lack of U.S. investment in Central America has stymied progress on rooting out corruption and violence, key factors pushing people in the region to migrate. Salazar says he agrees that improving those conditions through development assistance ought to be at the center of the United States’ agenda.

“Why do people leave Guatemala or other places and come north? It’s because they have no hope, they have no job, they’re hungry, they live in fear,” he said. “What we have to do is help create hope in those places.”

Salazar met with López Obrador on Tuesday. The Mexican president pitched him on a development program based in southern Mexico and Central America that would offer employment to would-be migrants in a large-scale reforestation project.

Mexican President Andrés Manuel López Obrador speaks Thursday during Independence Day celebrations in Mexico City. (Hector Vivas/Getty Images)

Other U.S. officials have raised doubts about the effectiveness of the idea, and some conservationists have raised concerns over its impact on the environment. But Salazar, drawing on his experience as secretary of the interior, said he was interested in the idea if it could improve conservation and provide jobs.

“I think there’s an opportunity for us to collaborate,” he said.

In a proposal prepared by Mexico’s government last month, officials suggested that the U.S. should incorporate López Obrador’s program into its regional development package aimed at deterring migration.

“The Mexican government would provide technical support in the project design process,” Mexican officials said.