AHC: Earlier U.S. Presidential Protection

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?
 
Couple succesful assassinations more? Perhaps Jackson is succesfully assassinated.

Jackson was actually one of the examples I was thinking of. But would a one off assassination do it? Lincoln was assassinated, bit it took two more slain Presidents before the Secret Service assumed it's protective mission.
 
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