What Part of Speech is the "Due" in "Due To"?

Came across this interesting usage note at Webster's New World's online dictionary. I hadn't realized that "due to's" function was disputed. Here's what I learned.

due to preposition: Because of

Usage Note: Due to has been widely used for many years as a compound preposition like owing to, but some critics have insisted that due should be used only as an adjective. According to this view, it is incorrect to say The concert was canceled due to the rain, but acceptable to say The cancellation of the concert was due to the rain, where due continues to function as an adjective modifying cancellation. This seems a fine point, however, and since due to is widely used and understood, there seems little reason to avoid using it as a preposition.