Kamala Harris highlighted her tough-on-migration stance during a long-anticipated trip Friday to the U.S.-Mexico border in Arizona, aiming to cover off a political vulnerability and rebut Donald Trump’s core campaign message that Democrats are soft on immigration enforcement.

“The United States is a sovereign nation, and I believe we have a duty to set rules at our border and to enforce them, and I take that responsibility very seriously,” Harris said in Douglas, Arizona, Friday evening…