The electoral process acts as a firewall against voter fraud. Going by popular vote will increase the incentive for voter fraud. Today, a fraudulent vote in New York can only affect matters in New York. Under popular vote, that same vote can affect votes in 49 other states. If you had a state like California which is heavily run by one party (Democratic, in this case), they could wink and nod as thousands of people stuff the ballots throughout the state.