When Did We Stop Being Christians?

I have a historical question that hopefully someone out there can help me figure out. When was the first time that Mormons were excluded from Christianity by Evangelicals or others? I have a feeling that it is pretty late, like sometime in the second half of the 20th century. Critics complain that Mormons have recently begun to emphasize their “Christianity”, but my theory is that Mormons have only recently done so because the charge that Mormons aren’t Christians is just as recent. Can anyone confirm or deny this?

The EoM article on “Anti Mormon Publications” is about the closest thing that we have to a history of this literature, and I don’t see anything there that indicates that early charges against Joseph Smith, or later charges against polygamy ever included the accusation that Mormons weren’t Christians. Heretics, yes, but not Christian? I just don’t think that they were doing this. I would love it if someone could provide a set of references to some material before the 1950’s that explicitly says that Mormons aren’t Christians.

A secondary question is why these charges arose at the time that they did, especially if it is as late as I think that it is. What was at stake or what had changed to begin to emphasize this specific accusation?