broad set of verbs that can constitute it, including intentionally harassing, intimidating, threatening, misleading, or corruptly persuading someone in order get that person to lie, alter evidence, or refrain from testifying. Includes, but doesn't need to include, physical threats.

I don't think there is any sort of exception for threatening someone in a manner that isn't on its own illegal (e.g., "If you testify, I'll mail this evidence of your brother robbing a bank to the cops"). That is still obstruction, because you are using it to threaten the person and prevent or alter testimony.