Source, please.
The source was mentioned in the post, William F. Buckley’s “National Review”.
“Put another way, the Catholic swing vote has more than any other decided the winner, just because it is of such significant numbers. No Democrat since 1952 (except for Clinton in 1992) has ever won the White House without a majority of the Catholic vote.”
“In some states, as noted above, Republicans do not have to win a majority of the Catholic vote to carry the state; they need only hold down the Democratic Catholic majority by two or three percentage points.”
Did you do a search for “national review Percentage of Catholic Vote for Presidential Winners”?