So, he's technically Jewish. Does that count? There's probably a number of Christians as well as Jews in Congress that are not practicing.
Web sources for "who is Jewish" will always cast the widest net possible. If someone identifies as Jewish
neither religiously nor culturally and was raised in a diverse family, I don't think it's appropriate to define him as such by his mother's ancestry. Russ Feingold is non-religious, to my knowledge, but still identified with his cultural heritage. Not that his last name gives him much of a choice.