I just recently mostly read (partly skimmed) a biography of old Joe Kennedy, which was very sympathetic, probably too much so, but it more or less made the point that Joe became a Democrat because the Republicans would not accept him.
The Republicans wouldn't accept him? In what way? He could have registered as a Republican w/o anyone knowing if he wanted to. My guess is that the GOP wouldn't take a bribe with a guarantee of something like Ambassador to the Court of King James as payback.
Old Joe was a philandering, whiskey smuggling limosine gangster. I'm glad he wasn't a Republican.
So far as Republicans are concerned, Joe wasn't the first "outsider" to see that he could have more power and influence in the Democratic Party. That goes on even now, and the GOP was much more the WASP party in those days.
Two cautions: 1) in those days people did vote more on the basis of ethnicity than ideology -- certainly whether you voted for Herbert Hoover or Al Smith was more about religion and ethnicity than about political philosophy -- and 2) if that's the David Nasaw biography, many people have criticized it for being a whitewash or a puff piece.