Wayne L. Cowdrey, et al. suggest that the Spaulding theory began when Orson Hyde preached in Conneaut in 1832.
Wayne L. Cowdrey, Howard A. Davis, and Arthur Vanick, "Introduction," in Who Really Wrote the Book of Mormon? The Spalding Enigma (St. Louis: Concordia Publishing House, 2005), 33
All things considered, the Spalding Enigma can be said to have begun on Sunday, February 12, 1832, when boyish-looking 27-year-old Mormon "High Priest" Orson Hyde "preached in Salem Village" (now Conneaut), then held a second meeting the next day at which, by his own account, some people became "excited."
Mormon missionary strategy at the time appears to have been to go into a town and conduct a reasonably generic "Christian" service at their first public meeting, placing strong emphasis on the biblical Old and New Testaments while making only minimal references to The Book of Mormon. Only after this had been successfully accomplished were interested parties and perspective converts then invited to a second meeting at which "the congregation was addressed on the coming forth of The Book of Mormon." Thus it may be reasonably presumed that it was Orson Hyde's second meeting in Conneaut (February 13, 1832), which is of primary concern here...
According to Judge Aron Wright, Nehemiah King's friend and associate, as soon as Orson Hyde finished his presentation, King left the hall and stated Hyde had preached from the writings of Solomon Spalding. Whether this was the cause of some people having become "excited" as mentioned in Hyde's journal remains unclear, but it is almost certainly the event that started talk in the area to the effect that Spalding's old romance about the Indians and the Lost Tribes of Israel and somehow been resurrected as the supposed Golden Bible of a new religion. Thus it would appear that such talk had circulated around the Conneaut region for more than a year and had already been accepted as truth by many of the area's older residents by the time anyone from the outside became interested enough to investigate further.