Argentina’s Supreme Court has upheld a six-year prison sentence against Cristina Fernández de Kirchner, the polarizing former president and one of the country’s most influential politicians, barring her from public office for life.

In the short-term, the decision may enable libertarian President Javier Milei to cement his power base in the October midterm vote, but over the long run it could help revamp the Peronist movement, w...