The Northeast and Midwest are heavily Catholic, and are also heavily UNION. These union members voted largely Democrat, because they considered the Republican party to be the party of the ‘bosses’, and the Democrat party to be that of the ‘workers’. The Democrats, being for the ‘little guy’, would always protect them. This attitude is hard to shake, because there have been generations of union workers, and their party loyalty was passed down through their families. Also, since John F. Kennedy was the first Catholic president, and he was a Democrat, that only cemented that attitude in peoples’ minds.
What they don't understand is that union bosses are all about extracting wealth from the union and care just about as much for the workers as the Democrats, which is nothing. They want the power.