Pope Francis arrives in Mexico on Friday, Feb 12, for a six-day visit that will end with a highly symbolic and potentially controversial act: the pontiff taking a stand on the fortified U.S. border to show solidarity with the migrants trying to cross it, the Wall Street Journal reports.

The pope’s gesture comes at the height of a rancorous U.S. political season in which immigration has become a hot-button issue.

Two leading candidates for the Republican nomination, Donald Trump and Ted Cruz, have vowed to build a wall along the nearly 2,000 miles of the U.S.-Mexican border. Trump has said he would forcibly deport up to 11 million illegal migrants from the U.S.

The pope will hold a cross-border Mass in Ciudad Juárez on Feb 17 just 90 yards from the U.S. frontier. Some 200,000 people are expected to attend on the Mexican side and an additional 50,000 across the Rio Grande in Texas.

“This is one community despite the fence,” Vatican spokesman Federico Lombardi told reporters on Tuesday. “I think it will be moving to see this single community even though it is located on two sides of the border.”

Before the Mass, the pope is expected to ride to the border line and offer a prayer for migrants, including those fleeing war in the Middle East and escaping deadly street gangs in Central America, the Journal says.

The son of Italian immigrants to Argentina, the pope has been an outspoken advocate for migrant rights during his three years as leader of the world’s one billion Catholics.

More than half a million Central American migrants—the vast majority from El Salvador, Guatemala and Honduras—were detained trying to cross the U.S. border during the past three years. That compares with nearly 200,000 the previous three years. An unknown number of migrants made it through.

Gang violence has placed the three countries—known collectively as the Northern Triangle—among the world’s deadliest corners. With a 72% spike in killings last year, El Salvador has overtaken neighboring Honduras as the world’s most murderous country not at war, with a per capita homicide rate 20 times that of the U.S.

Pope Francis’ defense of migrants echoes that of his predecessors, but he has given unprecedented priority to the topic and used more forceful rhetoric.

In July 2013, on his first trip outside Rome as pope, Pope Francis traveled to the southern Mediterranean island of Lampedusa, a major entry point for undocumented migrants into Europe. There, he condemned a “globalization of indifference” he saw epitomized by the deaths of those who have drowned on the sea crossing from North Africa.