It was simply common knowledge at the time. They didn’t realize that a definition might ever be needed, it was obvious to them. Federilist 68 talks about it. The founders considered it a matter of National Security.
Alexander Hamilton:
“Nothing was more to be desired than that every practicable obstacle should be opposed to cabal, intrigue, and corruption. These most deadly adversaries of republican government might naturally have been expected to make their approaches from more than one querter, but chiefly from the desire in foreign powers to gain an improper ascendant in our councils. How could they better gratify this, than by raising a creature of their own to the chief magistracy of the Union? But the convention have guarded against all danger of this sort, with the most provident and judicious attention.”
Also, WHY was it then so necessary to provide a special Senate resolution for McLame???