This week's Democratic primary election for Philadelphia district attorney could presage outcomes in the 2022 and 2024 elections, but not in the way the winner would like. Foreigners are often surprised that Americans elect most of their local prosecutors -- and that these prosecutors can become major forces in public force. But we do and they can. Thomas Dewey, as a 38-year-old Manhattan district attorney, was a major presidential candidate in 1940 and was the Republican nominee in the following two elections. Pat Brown got his political start as San Francisco DA, and he and his son, Jerry, were governors...