James Curley (and his rival, Honey Fitz, JFK's grandfather) were professional Irish politicians (Boston mayors I'm barely old enough to remember) and corrupt. Adam Clayton Powell was a professional Negro politician (US Rep from Harlem back in the day) and corrupt. JFK was a new sort of Irish-Catholic politician, who didn't run as either.
I think the lesson is that a politician who runs as a representative of a group can often get away with corruption. And it may take a couple generations for the "group" to melt enough in the American pot.