Didn't Boston start voting Dem before almost every other major city?
That's because of the Irish who lived there. They always voted Democratic.
But then why did Boston vote for Harding and Coolidge?
Due tot he isolationst feelings of the Irish-Catholic Bostonains after the war. Nearly every family in Boston had a son go over to fight WWI, and a majority were killed and wounded. They responded to Harding's isolationist call.
For the 1924 campaign, the Irish voted against John W. Davis because they felt that the Democratic Party was controlled by the KKK and that is wyh Al Smith was denied the 1924 nomination.