Carissa Wilson began paying attention to Rick Santorum last fall, when her high school class in American government was assigned to watch several of the Republican presidential debates. About three weeks ago, she attended a campaign speech that Mr. Santorum delivered at her school near Dayton, Ohio. The next day, March 6, Miss Wilson cast the first vote of her life, balloting for Mr. Santorum in the Ohio presidential primary.

It hardly mattered to Miss Wilson that she is an evangelical Protestant — raised as a Baptist, attending a nondenominational church and in her senior year at Dayton Christian School — and that Mr. Santorum is a Roman Catholic. In her thoughts and action, she typifies a cultural and religious phenomenon in the 2012 election: the...