Other that Abraham Lincoln, no U.S. President had a formal protection division until after the assassination of President McKinley in 1901. This strikes me as very odd given how visible U.S. Presidents were, and how (in most cases) easily accessible they were (Thomas Jefferson would usually open the door to guests at the White House).
So what event/events could drive Congress to formalize Presidential protection prior to 1900?
So what event/events could drive Congress to formalize Presidential protection prior to 1900?