First of all, a direct democracy is something else, it is a system where citizens vote for laws and other issues directly instead of appointing representatives such as congress or the president. France is not a direct democracy, even though they elect their president through a direct vote (like all other democratic nations that exist today except the USA).

Second of all, the constitution leaves it up to the states to decide how to appoint their electors. It follows that any system chosen by that state is as constitutional as any other system.

I would also add that a major improvement to the current rotten system would be for states to appoint their electors proportionally, instead of winner-takes-all. That would solve by far the biggest problem with the current system, which is not that Maine gets more representation per citizen than California, but that voting Republican in California is entirely useless.