Wherever crowds gathered in the flat, lake-dotted swath of Florida between Orlando and Ocala, it seemed that Cheryl Hall was there: clad in red, white and blue, clipboard in hand, signing up new voters at the Mount Dora Arts Festival, the Ocoee Founders’ Day party, the Taste of South Lake fair in Clermont and more.

“She just seemed like a diligent, hard-working lady,” said Alan Hays, the supervisor of elections in Lake County, who appears in a photo on Ms. Hall’s Facebook page, his arm firmly clasped around her shoulder. “No one suspected anything was amiss until we got phone calls from people.”

Something was indeed amiss: Ms. Hall, a 63-year-old and very ardent Republican whose ranch house in Clermont sports life-size cutouts of Donald and Melania Trump and a MAGA poster in the window, was charged last week with 10 felony counts of submitting false voter registration forms. On at least 10 forms traced to Ms. Hall, officials said, the party affiliations of already-registered Democrats and Independents had been switched to Republican. More than 100 others that may be tied to her contained missing or bogus data such as wrong birth dates.

Ms. Hall was a canvasser for Florida First Inc., a recently created nonprofit that is financed at least in part by a dark-money group formed by Mr. Trump’s re-election campaign manager, Brad Parscale, and other Trump associates.