Something realized today is that this would be a good county to look at: Dubuque, Iowa. It's commonly known as having one of the largest white Catholic populations in the country, at about 80%. It voted:
Kennedy 63.30%
Nixon 36.64%
So for 83%, just about all non-Catholics in the county would've had to have voted for Nixon. For 72%, that would be about 70% for Nixon, maybe not entirely unfeasible, but consider that even in Sioux County of all places Kennedy broke 20% (way better than any modern Democrat can expect). Of course the 72% figure is possible if you consider that in places like South Texas the numbers for Kennedy were much higher...but that just kind of underscores the point that there was no Catholic voting bloc even in 1960.
In the end though Al is definitely right, if I had to estimate based on the 1960 map I'd say mid to high 60s though.
You would have to demonstrate that the Catholic population in Dubuque was representative of the country as a whole. Otherwise, what you've done is the equivalent of looking at Vermont's results in the 2008 election, and determining that whites voted overwhelmingly for Obama because Vermont is one of the whitest states in the country.
Poll
