If he leaves the Democratic Party, he will take a journey many observing Catholics have already taken.
As a Catholic, you have to ask what tenets of the Catholic Church does the Democratic Party platform support. The answer is very little. Once a Catholic can separate in his mind the concept of real charity freely given from the costly government social programs collected at the point of a gun, there is nothing to keep him in the Democratic Party.
I still can't figure out what sort of Catholicism they are teaching in Massachusetts.
Some Catholics stay Democratic because they believe in the socialist Catholic liberation theology, taught by many Jesuit institutions around the country.