He isn’t doing away with the Electoral College - which would take an Amendment.
He is demanding that the states join a pact to allocate their Electoral Votes to whoever wins the popular vote, which simply takes state legislation.
Most states would have be insane to do this, but you never know. Insanity is pretty widely spread these days.
Fortunately, these days most governors and legislatures are ‘Pubbie, which reduces the chance.
I strongly suspect that the legislation, already passed by many states, would be deemed an unconstitutional “interstate compact.”
Someone may have more details than I do, but I would analyze this as follows:
State legislatures may select electors by whatever manner they wish, at least under The Constitution. (This assumes that the people of the state voting for the legislators know and don’t disapprove of the manner of selection.)
But if the “popular vote” provision is only effective if enough other states use the same criteria, then it’s in effect an “interstate compact.”
And of course it appears that mostly “Democrat” states have adopted this measure. So if a Republican wins the “popular vote” but loses the Electoral College, it would help the Republican if in fact these laws were constitutional at all.
It’s not about the laws, though, for many. It’s just about power, the laws be damned. Scary.