You're thinking of the 3/5ths compromise, which is a separate though related thing to the Electoral College.
The EC tended to favor non-plantation slavery states (only Maine and Massachusetts had zero slaves at the time, though several others were close to zero). Virginia was by far the biggest state by population, and it had tons of slaves. Without counting its slave population, it would still have been the largest. The Electoral College, at least at the time, favored small, relatively slave-free states like Rhode Island and Vermont. The 3/5ths compromise does give some extra power to the larger slave states in the EC by way of added population, but not proportionate to what small states get out of it.
Federalist 68 gives the official reasoning behind the EC:
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. They have not made the appointment of the President to depend on any preexisting bodies of men, who might be tampered with beforehand to prostitute their votes; but they have referred it in the first instance to an immediate act of the people of America, to be exerted in the choice of persons for the temporary and sole purpose of making the appointment. And they have excluded from eligibility to this trust, all those who from situation might be suspected of too great devotion to the President in office. No senator, representative, or other person holding a place of trust or profit under the United States, can be of the numbers of the electors. Thus without corrupting the body of the people, the immediate agents in the election will at least enter upon the task free from any sinister bias. Their transient existence, and their detached situation, already taken notice of, afford a satisfactory prospect of their continuing so, to the conclusion of it. The business of corruption, when it is to embrace so considerable a number of men, requires time as well as means. Nor would it be found easy suddenly to embark them, dispersed as they would be over thirteen States, in any combinations founded upon motives, which though they could not properly be denominated corrupt, might yet be of a nature to mislead them from their duty.
Another and no less important desideratum was, that the Executive should be independent for his continuance in office on all but the people themselves. He might otherwise be tempted to sacrifice his duty to his complaisance for those whose favor was necessary to the duration of his official consequence. This advantage will also be secured, by making his re-election to depend on a special body of representatives, deputed by the society for the single purpose of making the important choice.
In other words, the EC existed precisely to stop someone like Donald Trump from taking office. Or more accurately, stopping a populist, corrupt nitwit was the reason it was justified to the American people at the time. Back then, they were trying to head off criticism of democracy from supporters of constitutional monarchy. That is, democracy would be vulnerable to takeover by a populist, corrupt nitwit.
Obviously, it has failed at its one job. That doesn't mean democracy itself is vulnerable. On the contrary, the EC is a specifically anti-democratic concept that has been abused by exactly the sort of populist, corrupt nitwit it was supposed to guard against.
