A candidate needs a majority of electoral votes — 270 — to become president. Trump has 306 electors pledged to him. So 37 would need to defect from him to put him below that majority threshold.VOX

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, The Federalist Papers : No. 68 http://avalon.law.yale.edu/18th_century/fed68.asp