The Southern Poverty Law Center has just put out a study of the most extreme fringes of American Catholicism. It isn't pretty:
Few Americans defended Mel Gibson's drunken rant about the evils of the Jews. But radical traditionalist Catholics did. A three-year investigation of this subculture by the Intelligence Report has found that these Catholic extremists, including the Gibsons, may well represent the largest population of anti-Semites in the United States. Organized into a network of more than a dozen organizations, scores of websites and several extremist churches and monasteries, radical traditionalists in the U.S. are preaching anti-Semitism to as many as 100,000 followers. A few, such as the lawyer for Terri Schiavo's family, Christopher Ferrara, are even movers and shakers in important right-wing Republican circles.
You can read the whole thing here.