In 1960 Sinatra was a huge Kennedy supporter who raised a ton of money for JFK. I believe he planned the inaugural celebrations as well. Showing their class, the Jack and Bobby then basically told ol' Blue Eyes to screw. Frank took it personally (as he did everything) and eventually turned Republican.
I didnt know he had turned Republican. I do think he had a piece of a certain Republican woman who has a son who is gay and strikingly resembles Sinatra.