As Vice President Kamala Harris vows to crack down on illegal immigration at the nation’s southern border, she has played up her experience as California’s attorney general pursuing criminal groups that traffic drugs, guns and people across the United States and Mexico.“I was the top law enforcement officer of the biggest state of the country, California — that is also a border state,” she said on Thursday at a town hall in Nevada. “I took on transnational criminal organizations.”Ms. Harris’s surrogates and allies say her push on transnational crime sheds light on how she might address challenges at the border....