MEXICO CITY—Pope Francis arrives in Mexico on Friday 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 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. Mr. 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.”